Old Irish-Gaelic Surnames
Translation from Gaelic to English
(sorted by Anglicized Surname - disregarding O and Mac prefixes)

Gaelic Name - Anglicized name - Early County Origin - Other Roots
Mac an Bhreitheamhan - (Mac) Abraham, Abrehan, Judge - Co Cork
Ó hEachthianna - Ahern or Hearne - Co Clare - the Dal gCais (e.g. Brian Boru)
Mac an tSaoir - MacAteer - Ulster, (St. Ciaran)
Mac Amhalghaidh - MacAuley or MacAwley - Co Offaly/Westmeath, lords of Calry
Mac Amhlaoibh - MacAuley or MacAwley - Co Fermanagh - branch of MaGuire
Mac Amhlaoibh - MacAuliffe - Co Cork - branch of MacCarthy
Mac Árdghail - MacArdle - County Monaghan, Armagh and Louth of Oriell
Ó Banain - O'Banan or O'Bannion - Co Fermanagh
Ó Banain - (O) Bannon or O'Bannion - Co Tipperary
Ó Beaglaoich - Begley - Co Donegal
Ó Birn - (O) Beirne - Co Roscommon - chiefs of Tir Briuin
Ó hAimheirgin - (O) Bergin - Co Offaly - chiefs of Geashill
Ó Beolláin - (O) Boland - Co Sligo - Uí Fiachrach
Ó Beolláin - (O) Boland - Co Clare - Dalcassian
Ó Baoghill - (O) Boyle - Co Donegal
Ó Baoighealláin - (O) Boylan - Fermanagh/Monaghan - same stock as O'Flanagans
Mac Bradaigh - Brady - Co Cavan
Ó Brollacháin - (O) Brallaghan or Bradley - Co Tyrone/Derry - Cinel Feradaigh
Mac Braoin - (Mac) Breen - Ossory, Co Kilkenny
Ó Braoin - (O) Breen - Westmeath/Offaly - Lord of Brawney
Mac an Bhreitheamhnaigh - (Mac) Breheny, Judge - Co Sligo/n. Roscommon
Mac Branain - MacBrennan - Co Roscommon - chiefs of Corca Achlann
Ó Braonain - (O) Brennan - Co Kilkenny, East Galway, Westmeath and Kerry
Ó Breislein - (O) Breslin - Co Donegal, Cenel Enda, chiefs of Fanad
Ó Bric - (O) Brick - Co Waterford - of the Decies
Ó Bruic - (O) Brick - Co Kerry - originally of Thomond
Mac Giolla Bhride - MacBride or Kilbride - Co Donegal
Ó Briain - O'Brien - Munster (Thomond) - the Uí Toirdealbhaigh, part of the Dal gCais
Ó Bruadiar - (O) Broder or Broderick - Co Cork, Limerick and Galway
Ó Brógáin - (O) Brogan - Co Mayo - the Uí Fiachrach
Ó Brolaigh - (O) Brolly, Brally, Brawley - Co Donegal/Derry - Cinel Oenghusa
Ó Bróithe - (O) Brophy - Co Kilkenny, Laois
Ó Brosnacháin - (O) Brosnan - Co Kerry
Ó Buachalla - Buckley - Co Tipperary/Offaly
Ó Bogaigh - Buggy - Co Kilkenny/Wexford (origin unknown)
Mac Conboirne - Burns - N. Connaught - mainly of Scottish origin
Ó Broin - (O) Byrne, Burns - Co Kildare/Wicklow - Uí Broin sept of Crioch Branach
Mac Cába - MacCabe or McCabe - Co Cavan, etc. - a Breffny sept from the western isles of Scotland
Mac Gafraidh - (Mac) Caffrey - Co Tyrone/Fermanagh - branch of the Maguires
Ó Cathain - O'Cahan or Kane - East Donegal - Cenel Eoghain
Ó Cathail - (O) Cahill - Co Galway - chief of Kinelea
Ó Cathail - (O) Cahill - Co Kerry and also in Tipperary
Ó Ceallacháin - (O) Callaghan - Co Cork - the Eoghanacht, descent from Ceallachan
Ó Cathaláin - (O) Callan - Co Monaghan/Louth - named from a king of Fernmag.
Mac Giolla Laoire - (Mac) Callery, Colleary, Collary - Co. Sligo & north Connacht
Mac Cathmhaoil - Campbell - Co Tyrone - Cenel Fearadhaigh Theas of Cenel Eoghain
Mac Cailein - Campbell - Co Donegal - Scottish gallowglass of Cenel Eoghain
Ó Ceanndubháin - (O) Canavan - Co Galway, physicians to O'Flaherty of Connemara
Mac Cuindlis - MacCandless - Co Down
Mac Anna or Canna- MacCann - Co Armagh - lords of Clanbrassil (Cenel Eoghain)
Ó Canain - (O) Cannon - Co Galway - the Uí Maine, same stock as O'Madden
Ó Canannain - (O) Cannon - Co Donegal - Kings of Cinel Conaill (pre-1200)
Ó Caithniadh - (O) Canny - Co Mayo - Uí Fiachrach Muaidhe, chiefs of Erris
Ó an Cháintigh - (O) Canty - west Co Cork
Ó Cairbre - (O) Carbery - Co Westmeath - chiefs in Clonlonan
Ó Ciardha - (O) Carey - Co Kildare - southern Uí Neill, lords of Carbury
Ó Cearmada - (O) Carmody - east Co Clare
Mac Cearnaigh - (Mac) Carney or Kearney - Co Meath and Monaghan/Armagh
Ó Cearrnaigh - (O) Carney - Co Mayo - Uí Fiachrach
Ó Cairealláin - (O) Carolan - Co Derry - chief of Clan Diarmada
Ó Cearbhalláin - (O) Carolan - Co Cavan/Monaghan
Ó Cairre - (O) Carr and (O) Carry - various, e.g. of Cenel Aengusa
Ó Cearbhaill - (O) Carroll - Co Offaly and Tipperary - lords of Eile (Ely)
Ó Cearbhaill - (O) Carroll - Co Monaghan and Louth - chiefs of Oriel
Ó Cearbhaill - (O) Carroll - Co Leitrim/Sligo - chief of Calraighe
Ó Cearbhaill - (O) Carroll - Co Kilkenny - chief of northern Gowran
Ó Cearbhaill - (O) Carroll - Co Kerry - chief of Locha Lein
Mac Cearbhaill - (Mac) Carroll or MacCarvill - Ulster
Mac Artian - MacCartan - Co Down - chiefs of Kinelarty
Mac Carthaigh - MacCarthy - Munster (Tipperary) - the Eoghanacht
* MacCarthy Mor - South Kerry
* Dunhallow MacCarthys - Northwest Cork
* MacCarthy Riabhach - Carberry, Southwest Cork
* MacCarthy Muskerry - Cork/Kerry border
Ó Cathasaigh - (O) Casey - various, including lords of Suaithni (co. Dublin)
Mac Cathasaigh - MacCasey or Casey - Co Louth/Monaghan
Ó Caiside - Cassidy - Co Fermanagh
Mac Fhlannchaidh - (Mac) Clancy - Co Clare - Dal gCais, branch of MacNamara
Mac Fhlannchaidh - (Mac) Clancy - Co Leitrim - chief of Dartry (Rosclogher)
Ó Cleireigh(Clery) - Clark(e) - Connacht, Donegal - chiefs of Uí Fiachra Finn
Ó Cléirigh - (O) Clery or Cleary - Co Galway - Tigherneach Ua Cleirigh, lord of Aidhne
Ó Cleirchín - (O) Clerkin, Clerk, Cleary - Co Limerick - Uí Fidhgeinte, Uí-Cairbre
Ó Clochasaigh - (O) Cloghessy, Clohessy - Clare Clare, east Limerick. Dalcassian
Mac Cluin - (Mac) Clune - Co Clare
Mac Bhloscaidh - McCluskey - Co Derry - a branch of the O'Cahans
Ó Cobhthaigh - (O) Coffey - Co Cork - same stock as O'Driscolls
Ó Cobhthaigh - (O) Coffey - Co Roscommon - Uí Maine branch of the O'Maddens
Ó Cobhthaigh - (O) Coffey - Co Westmeath
Mac Cagadhain - Cogan - Co Leitrim
Ó Colmáin - Coleman - Co Sligo, barony of Tireragh - Uí Fiachrach
Ó Clúmháin - Coleman or O'Colman - Co Sligo/Cork - descendant of Clumhan
Mac Colghain - (Kil) or (Mac) Colgan - Co Offaly - same stock as O'Dempsey and O'Dunne
Ó Colghain - (O) or (Mac) Colgan - Co Derry and also of Co Donegal
Ó Coileáin - Collins - Co Limerick and later West Cork - lords of Connello
Ó Cuilleáin - Collins - West Cork - Corca Laoidhe
Ó Concheannain - (O) Concannon - Co Galway - Uí Briúin in Uí Maine, lords of Uí Diarmada
Ó Caoindealbháin - (O) Conlan, Quinlan - Co Meath, princes of Uí Laeghari
Ó Connalláin - Conlon, Conlan or Connellan - Co Galway and Roscommon
Ó Coinghiolláin - Conlon - Co Sligo
Mac Conghaile or Ó Conghaile- (Mac) Conneely - Co Galway (often confused with "Connolly")
Ó Conaill - O'Connell - Co Kerry - chief of Magunihy
Mac Dhomhnaill - Connell and MacConnell - Ulster - the Clan MacDonald
Ó Conghailaigh or Ó Conghaile - (O) Connelly or Connolly - SE Co Galway - Síl nAmnchadha Uí Máine
Ó Conghalaigh or Ó Conghaile - (O) Connolly - Co Fermanagh and Monaghan
Ó Coingheallaigh - Connolly - West Cork
Ó Congalaig - (O) Connolly - Co Dublin/Meath
Mac Canann - Mac Connon - Co Monaghan
Conchobhair - (O) Connor - Co Offaly - descent from Cathaoir Mor
* O'Conor Don, Roe, & Sligo - Connacht, Co Roscommon
* O'Conor Faly - Co Offaly
* O'Conor of Corcomroe - North Clare
* O'Connor Kerry - Co Kerry and also in Keenaught, Co Derry
Ó Maolconaire - (O) (Mul)Conry or Conroy - Co Roscommon
Ó Conraoi - (O) Conry or Conroy - Co Roscommon/Galway - Uí Maine
Mac Conraoi - (Mac) Conroy or King - Co Galway - lord of Delvin of the Two Lakes
Mac Consaidín - MacConsidine - Co Clare - Dal gCais, a branch of O'Brien
Mac Conmhaoil - MacConville or Conwell - Co Armagh (Oriel)
Ó Conbhuidhe - (O) Conway - Co Sligo
Ó Connmhacháin - (O) Conway - Co Mayo - same stock as O'Hara
Mac Connmhaigh or Connmeadha - (Mac) Conway - Thomond
Mac Conmidhe - (Mac) Conway or MacNamee - Co Derry and Tyrone
Mac Congail - Conwell or MacConwell - Co Down
Ó Cuagain - Coogan - Galway/Roscommon - the Uí Maine
Mac Giolla Chomhgaill - (Mac) Cool - Donegal
Mac Cuille or Mac Giolla Chuille - Cooley - Connacht, Roscommon, Clare, Galway.
Ó Cuana - Cooney - Co Tyrone, then north Conacht
Ó Culachain - Coolihan or O'Cuolihan - Co Mayo - the Uí Fiachrach
Mac Corcrain - (Mac) Corcoran - Co Offaly/Tipperary, chief of Clan Ruainni
Ó Corcráin - (O) Corcoran - Co Fermanagh
Mac Cormac - MacCormack - various
Ó Corraidh or Ó Corra - (O) Corr or Corry - Co Tyrone-Fermanagh
Ó Corragáin - (O) Corrigan - Co Fermanagh - same stock as Maguire
Ó Cosraigh - (O) Cosgrave - Co Wicklow, later Wexford
Ó Cosraigh - (O) Cosgrave - Co Galway - Uí Maine
Ó Cosraigh - (O) Cosgrave - Co Monaghan - chiefs of Fir Rois
Mac Oistoghín - (Mac) Costigan - Co Laois - a branch of Fitzpatrick
Mac Oitir - (Mac) Cotter - Co Cork
Mac Cochláin - (Mac) Coughlan - Co Offaly - Dal gCais, chief of Delvin MacCoughlan
Ó Cochláin - (O) Coughlan - Co Cork
Mac Coibheanaigh - Coveney - Co Kilkenny (Ossory) - in Crannagh barony
Mac Giolla Chomhgaill - MacIlhoyle or Coyle - Donegal
Ó Cadhain - Coyne, Kyne or Coen - Co Mayo - the Uí Fiachrach
Mac Ruidhri - MacCrary or MacCreary - Unknown - variant of MacRory?
Craobhach - Creagh - Co Clare - branch of the O'Neills of Clare
Ó Croidheáin - (O) Crean - Co Donegal/Sligo - Cenel Eoghain
Ó Creacháin - (O) Crehan - Co Mayo - the Uí Fiachrach
Ó Criocháin - (O) Crehan or Creighton - Co Tyrone- an Oriel sept
Mac Conchruachan - (Mac)Croghan - Co Roscommon
Mac Criomhthainn - MacCrohan - Co Kerry
Ó Croinin - Cronin - Co Cork
Ó Cruadhlaoich - Crowley - Carbery, Co Cork - Uí Briuin Ai
Ó Cuidighthigh - Cuddihy - Co Cork and Tipperary
Ó Cathaláin - (O) Culhane or Cahalane - Co Limerick - chiefs of Owney Beg
Ó Cuilinn - (O) Cullen - Co Wexford and Wicklow
Ó Cuileannáin - (O) Cullinan - Co Clare
Ó Cuileannáin - (O) Cullinan or Cullen - Co Donegal - of Mullinashee
Ó Cuileannáin - (O) Cullinane - Co Cork - Corcu Liadhe
Ó Coimín - (O) Cummins, Commons - Co Mayo
Mac Coinín - (Mac) Cunneen - Co Mayo
Ó Coinín - (O) Cunneen or Rabbit - Co Offaly and of Thomond
Ó Connagáin and Mac Cuinneagain - Cunningham - Co Sligo - Uí Fiachra
Ó Connagáin and Mac Cuinneagain - Cunningham - Galway/Roscommon - Uí Maine
Mac Toirdhealbhaigh - MacCurley, Curley or Terry - Connacht
Ó Curráin - (O) Curran - SW Connacht - Uí Maine
Ó Corraidhin - (O) Curreen or Curren - Co Leitrim
Mac Curtain - (Mac) Curtin - Co Clare
Mac Cosraigh - MacCusker - Co Tyrone - ernaghs of Clones
Ó Comhraidhe - (O) Corry and Curry - Co Clare
Ó Comhraidhe - (O) Curry - Co Westmeath - chiefs of Moygoish
Ó Dalaigh - (O) Daly - Co Westmeath - Uí Maine branch of the southern Uí Neill
Ó Dorchaidhe - (O) Darcy - Co Mayo and Galway - also of Norman origin
Ó Duibhdabhoireann - (O) Davoren - Co Clare
Ó Deághaidh - (O) Dea - Co Clare - of Inchiquin
Ó Dulchaointigh - (O) Delahunty - Co Tipperary/Kilkenny
Ó Dubhshláine - (O) Delany - Co Laois
Ó Diomasaigh or Dimmussaig - (O) Dempsey - Clanmalier between Co Laois and Offaly
Ó Duineachdha - (O) Dennehy or Denny - Co Cork
Ó Duibhdíorma - (O) Dermond, Dermott, Darby - chiefs of Breadach in Inishowen
Mac Diarmada - MacDermot - Co Roscommon/Sligo - of Mag Luirg (Moylurg)
Ó Duibheamhna - (O) Devany - Co Armagh/Down - ancient chiefs of Ui Breasail
Ó Duibhghiolla - (O) Devilly - Co Galway - the Uí Fiachrach Aidne
Ó Duibhghiolla - (O) Devilly - Co Galway - the Síol Anmchadha
Ó Daimhín - (O) Davin or Devine - Co Fermanagh - lord of Tirkennedy
Ó Doibhilin - (O) Devlin - Co Tyrone and Sligo - lords of Muinter Devlin, Cenel Eoghain
Ó Doibhilin - (O) Devlin - Co Sligo - chiefs in Corran, standard bearer to O'Connor
Ó Dubhuidhe - (O) Devoy or Deevy - Co Laois - 7 Septs of Leix, lord of Cremthainn
Ó Duilleain - Dillon - Co Westmeath, Longford and Mayo - Norman origin
Ó Daighnáin - Dinan - Co Clare and Cork
Ó Duinnín - (O) Dinneen, Dinan - Co Cork - the Corcu Laiodhe
MacDail re Deachair - (Mac) Dockery - Co Roscommon - Clan Dail re deacair (Sil Murray)
Ó Dubhchonna - Doheny or Downey - Co Cork - the Corcu Laoidhe
Ó Dochartaigh - Doherty or Dougherty - Co Donegal - Cenel Conaill, lords of Inishowen
Ó Dubhhláin or Dobhailen - Dolan - S. Roscommon and E. Galway - the Uí Maine
Ó Domhnallain - Donlan - S. Galway - the Uí Maine
Mac Donnchadha - MacDonagh - Co Cork - lords of Duhallow, branch of MacCarthy
Mac Donnchadha - MacDonagh - Co Sligo - lords of Corran, a MacDermot branch
Ó Donnagáin - (O) Donegan - Co Cork - of Muscraige Tri Maighe
Ó Donnagáin - (O) Donegan - Co Westmeath - of Críoch Dungan
Mac Duinnshléibhe - (Mac) Donlevy or Dunleavy - Co Down (Ulidia)
Mac Domhnaill - MacDonnell - Co Antrim - of Clan MacDonald
Mac Domhnaill - MacDonnell - Co Fermanagh - of Clankelly
Mac Domhnaill - MacDonnell - Clare/Limerick (Thomond)
Ó Domhnaill - O'Donnell - Co Donegal - the Northern Uí Neill
Ó Domhnaill - O'Donnell - west Clare and east Galway - the Uí Maine
Ó Domhnalláin - O'Donnellan - Co Galway - the Uí Maine, lord of Clan Breasail
Ó Donnghaile - Donnelly - Co Donegal. later Tyrone - Cinel Eoghain
Ó Donnchadha - (O) Donoghue Mór - Co Cork, Kerry - Eoghanacht Locha Lein
Ó Donnchadha - (O) Donoghue of the Glen - Co Cork. Kerry - the Eoghanacht
Ó Donnchadha - (O) Donohoe - Galway, Cavan and various
Ó Donndubháin - (O) Donovan - Co Limerick, then SW Cork - chiefs of Carberry
Ó Duacháin - (O) Doohan - Co Donegal
Ó Duighlaich - (O) Dooley - Co Westmeath, then NE Tipperary - lords of Fertullagh
Ó Deoráin or O Deoradháin - (O) Doran - Co Laois - 7 Septs of Leix
Ó Deargáin - (O) Dorgan - Co Cork
Ó Dudbhchon - (O) Doughan - Co Cork - Corcu Laoidhe sept
Ó Dubhda - (O) Dowd - Co Mayo and West Sligo - Uí Fiachrach Muaide
Ó Dunlaing - (O) Dowling - Co Laois - 7 Septs of Leix
Ó Dúnadhaigh - (O) Downey - Co Galway - lords of Sil Anmchadha
Ó Dúnadhaigh - (O) Downey - Cork/Kerry/Limerick border - of Luachair
Ó Dubhghaill - Doyle - Co Carlow, Wexford and Wicklow (Norse origin?)
Ó hEidirsceoil - (O) Driscoll - South Kerry, then West Cork - Corca Laoighde
Ó Diubháin - Duane or Devane - Co Galway - erenaghs of Killursa
Ó Dubhthaigh - (O) Duffy - Septs from Co Donegal, Roscommon and Monaghan
Ó Dubhagáin - (O) Duggan - North Cork - Fir Maighe
Ó Dubhagáin - (O) Duggan - Co Galway - in the territory of the Uí Maine
Ó Diubhgeannáin - (O) Duigenan, Deignan, Dignam, ... - Co Roscommon, Leitrim - erenaghs of Kilronan
Ó Doinn or Duinn - (O) Dunn or Dunne - Co Laois - lords of Iregan
Ó Donnchadha - (O) Dunphy - Co Kilkenny - of Iverk barony
Ó Diubháin - Dwane, Devane or Duane - Co Meath - of Knowth
Ó Diubháin - Dwane, Devane or Duane - Co Cork - Corcu Laiodhe
Ó Duibhir or O Dubhuidhir - (O) Dwyer - West Tipperary - of Kilnamanagh
Mac Aodhagáin - (Mac) Egan - Uí Maine territory, then Ormond
Mac Giolla Domhaigh - MacEldowney - Co Derry
Mac Giolla Rua - (Mac) Elroy - Co Fermanagh
Mac Conchradha - MacEnchroe or Crowe - Co Clare
Mac Inneirghe - MacEnery - Co Limerick - Uí Cairbre, same stock as O'Donovan
Ó hAonghuis - Ennis - Co Offaly and Meath - variant of O'Hennessy
Mac Fhiodhbhuidhe - MacEvoy - Co Westmeath, Laois - one of the "7 Septs of Leix"
O'Faodhagáin- Fagan - E. Airgialla (Oriel) - also of Norman origin
Ó Fathaigh - Fahy - Co Galway - Uí Máine, of Pobal Muintir Uí Fhathaigh
Ó Fallamháin - (O) Fallon - Co Roscommon - sept of the Uí Máine (Clann Udach)
Ó Fallamháin - (O) Fallon - Co Meath - of Crich na Cetach
Ó Failbhe - (O) Falvey - Co Kerry - the Corca Duibhne
Ó Fhearghaill or Fergail - (O) Farrell - Co Longford - lords of Annaly
Ó Faircheallaigh - (O) Farrelly or Farley - Co Cavan/Meath
Ó Faracháin - (O) Farren - Co Donegal - Lord of Finross
Ó Fearáin - (O) Fearon - Co Armagh (Oriel) - the Cenél Eoghain
Ó Fiannaidhe - (O) Feeney - Co Sligo/Mayo - sept of the Uí Fiachrach
Ó Fidhne - (O) Feeney - Co Galway/Roscommon
Ó Fithcheallaigh - (O) Fehilly, Field - Co Cork - sept of the Corca Laoidhe
Ó Fiachna - - (O) Feighney, Feeheny, Hunt - Co Roscommon/Sligo - of Síol Muireadhaigh
Ó Fiachra - - (O) Feighry, Feehery, Hunt - Co Tyrone
Ó Fionnaláin - (O) Fenelon - Co Westmeath - chiefs of Delvin (Delbna)
Ó Fiangusa - (O) Fennessy - Co Tipperary/Waterford - chiefs of a district near Cashel
Ó Fearadhaigh - (O) Ferry, Farry - Co Donegal - Cenél Conaill
Ó Fionnachta - (O) Finaghty, Finerty - Co Roscommon - of Síol Muireadhaigh
Ó Fionnachta - (O) Finaghty, Finerty - Co Galway/Roscommon - Uí Maine chiefs of Clanconway
Ó Fionnáin - (O) Finan - Co Mayo - Uí Fiachrach Muaighe chiefs of Coolcarney
Ó Fionn - (O) Finn - Co Sligo - chiefs of Calry
Ó Fionn - (O) Finn - Co Galway - erenaghs of Kilcolgan
Ó Fionn - (O) Finn - Co Monaghan - of Fir Rois
Ó Fionnagáin - Finnegan - Roscommon/Galway
Ó Fionnagáin - Finnegan - Cavan/Monaghan/Louth - a Breifne sept
Ó Finntighearn - Finneran - Co Westmeath? - chiefs of Uí Mella
Mac Fionbhair - Finnevar - Co Leitrim - chiefs of Muintir Greadain
Mac Giolla Mo-Cholmog - Fitz Dermot - Uí Dunchada of Dublin/Kildare
Mac Giolla Phadraig - Fitzpatrick - lords of Upper Ossory (Laois/Kilkenny)
Ó Flaithbheartach - (O) Flaherty - Co Galway - Lord of Moycullen and Iar-Chonnacht
Ó Flannagáin - Flanagan - Roscommon & Offaly
Ó Flannagáin - (O) Flanagan - Fermanagh - Cenel Cairbre of Tuath Ratha (Toora)
Ó Flannagáin - (O) Flanagan - Monaghan - Clan Ceallaigh - the MacDonnels of Clankelly
Ó Flannabhra - Flannery - Co Mayo - the Uí Fiachrach
Ó Flannabhra - Flannery - Co Limerick - the Uí Fidhgheinte, chief of Dal Cairbri
Ó Flaithre - (O) Flattery - Co Down - lords of Lecale, kings of Ulidia
Ó Floinn - (O) Flynn - Co Cork - Muscraige Uí Fhloinn
Ó Floinn - (O) Flynn - Co Cork - Corcu Laoidhe
Ó Floinn - (O) Flynn - Co Roscommon
Ó Flainn - (O) Flynn - Co Antrim
Ó Fhloinn - (O) Lynn - Co Armagh - Clanna Rury, descent from Coll Uais
Ó Fhógartaigh- Fogarty - Co Tipperary, Eliogarty - Dalcassian
Ó Foghladha - Foley - Co Waterford
Mac Giolla na Naomh - Ford - South Connacht
Mac Conshnámha - Forde or MacKinnawe - Co Leitrim - chiefs of Muintir Kenny
Ó Fuaráin - Forde or Foran - Co Cork
Macgiollarnath - Ford - southern Connemara
Ó or Mac an tSionnaigh - Fox - Co. Meath - nickname of a chief of Teffia
Ó Firghil - (O) Friel - Co Donegal - descent from Eoghan, brother of St. Columbcille
Ó Gamhna - Gaffney or Caulfield - Ossory (Laois/Kilkenny)
Mac Carrghamhna - Gaffney or MacCaron - Co Cavan and also Roscommon
Mac Conghamhma - Gaffney or Caulfield - S Galway - Uí Fiachrach Aidhne
Ó Gallchobhair - (O) Gallagher - Co Donegal - Cenel Conaill
Mac Giolla Riabhaigh - Gallery, MacAreavy, Gray - Co. Clare & elsewhere
Ó Gealbháin - (O) Galvin - Co Clare
Ó Gaibhtheacháin - (O) Gahan - Co Mayo - the Uí Fiachrach Muaide
Mag Fhionnáin - (Mac) Gannon - Co Mayo
Ó Gadhra - O'Gara - Co Sligo/Roscommon - chiefs of Luighne Connacht
Mag Fhearadhaigh - (Mac) Garry - Co Roscommon/Leitrim
Ó Gairbhith - (O) Garvey - southern Co Armagh, then Down
Ó Gairbhín - (O) Garvey or Garvin - Co Meath, then Mayo - So. Uí Neill
Ó Garbháin - (O) Garvey or Garvan - Co Cork/Kerry - same stock as O'Moriarity
Mac Gairbhín - (Mac) Garvey - Co Donegal
Mag Aodha - MacGee - Co Westmeath - of Muintir Tlámáin
Mag Aodha - MacGee - Donegal/Tyrone border
Mag Aodha - MacGee - Also of Scottish extraction, with Irish origins
Mag Aonghusa - Magennis or (Mac) Guinness - Co Down - the Dal Araidhe, lords of Iveagh
Mag Eochagáin - (Mac) Geoghegan - Co Westmeath - so. Uí Neill, chiefs of Kinalea
Mag Oireachtaigh - (Mac) Geraghty - Co Roscommon/Galway - Uí Briuin Ai (in Uí Maine)
Mac Giolla Phóil - (Mac)Gilfoyle or Powell - Co Offaly
Mac Giolla Mochuda - MacGillycuddy - Co Kerry - an O'Sullivan Mór branch
Ó Glaimhin - Glavin - Connacht - the Hy Fiachrach
Ó Glesáin - Gleeson or O'Glissane - Tipperary - same stock as O'Donegan
Mag Fhloinn - MacGlinn - Roscommon/Westmeath
Ó Gradaigh - (O) Grady - Co Clare & Limerick - the Dal gCais
Mac Congail - MacGonigle - Co Donegal
Ó or Mac Gormain - Gorman - Co Laois - Uí Bairrche
Ó Gairmleadhaigh - (O) Gormley - Raphoe, Co Donegal - of Cinel Moen (their tribe name)
Mag Shamhradháin - MacGovern / Magauran - Co Cavan - descent of Eochaidh (O'Rourke)
Mac Gabhann - MacGowan or Smith - Co Cavan
Ó Grádaigh - (O) Grady - Co Clare and Limerick
Mac Raith - MacGrath - Co Donegal/Fermanagh
Mac Raith - MacGrath - Co Clare
Ó hUaithnigh - Greene or Green - Co Cork
Ó hUaithnigh - Greene or Green - Co Clare
Mac Glaisin and Mac Giolla Ghlais - Greene or Green - Ulster
Ó Griobhtha - Griffin - Co Kerry and Clare
Mac Carrghamhna. later Ó Gramhna - (O) Growney - Co Tipperary
MagUiginn or MacGuagáin - MacGuigan - Co Tyrone - Cenel Eoghain
Mac Aonghasa or Oengussa - (Mac) Guinness or Magennis - Co Down - the Dal Araidhe
Mag Uidhir - MacGuire or Maguire - Co Fermanagh - rulers of Fermanagh
Ó hÉidin - Hade, Haide, Hayde - Co Galway
Ó hAodhagáin - (O) Hagan or Aiken - Co Tyrone - chiefs at Tullahogue
Mac Céile - MacHale - Co Mayo - also of Welsh origin
Ó haileacháin - (O) Hallahan - Co Cork/Waterford
Ó hAileagáin - (O) Halligan - Co Louth/Armagh (Oriel)
Ó hAllmhuráin - (O) Halloran - Co Clare (Thomond) - same stock as McNamaras
Ó hAllmhuráin - (O) Halloran - Co Galway - Chiefs of Clann Fearghaile
Ó hAghmaill - (O) Hamill - South Tyrone - Cenél Eoghain (Cenél Binnigh)
Ó hÁinle - (O) Hanley or Hanly - Co Roscommon
Ó hAnluain - (O) Hanlon - Co Armagh - lords of Orior
Ó hAnnáin - O'Hannon - Co Clare/Limerick - the Dal gCais
Ó hAnrachain - (O) Hanrahan - Co Clare/Limerick - the Dal gCais
Ó hAnrachtaigh - (O) Hanraghty or Hanratty - Co Armagh and north Co Louth (Uí Meith)
Ó hEaghra or Egra - O'Hara - Co Sligo - lords of Luighne with the O'Garas
Ó hÍr - O'Hare - Co Armagh - Oriel sept of Ó hÉir
Ó hEarcáin - (O) Harkin, Harkan - erenaghs of Clonska, Co Donegal
Ó hAirt - (O) Hart - Co Meath - southern Uí Neill, Four Tribes of Tara
Ó hIongardail - Harrington - Co Cork and Kerry
Ó hArrachtain - Harrington or O'Harraughton - Co Galway
Ó hArtagáin - (O) Hartigan - Co Clare/Limerick - the Dal gCais
Ó hEidedín - Hayden or (O) Headon - Co Carlow
Ó hAodha - Hayes - various counties
Ó hAodha - (O) Hea - Co Cork, Corca Laoidhe
Ó hEilidhe - Healey - Co Sligo
Ó hEaladaighthe - Healey - Muskerry, Co Cork
Ó hEilidhe - (O) Healy - Co Sligo
Ó hÉalaighthe - (O) Healy or Healihy - Co Cork
Ó hÉignigh or O hEicnig - (O) Heany or MacAnenay - Co Fermanagh - Kings of Fermanagh
Ó hEachighearna - Hearne or Ahern - Co Clare - the Dal gCais
Ó hEadroman - (O) Hederman, Hetherman, Heatherman - Co Clare/Limerick, east Cork
Ó hIfearnáin - (O) Heffernan - Co Clare/Limerick - chiefs of Owneybeg
Ó hEigceartaigh - Hegarty - Co Donegal & Derry - Cenel Eoghain
Ó hAichir - (O) Hehir or (O) Hare - Co Clare - Uí Fidhgheinte, lords of Magh Adhair
Ó hAonghusa - Hennessy or Henchy - Co Offaly - lords of Clan Cholgain
Ó hAonghusa - Hennessy - Co Dublian/Meath - chief of Gailenga Beg
Ó hAonghusa - Hennessy - Co Cork - Corca Laoidhe
Ó hInneirghe - Henry or O'Henery - Co Tyrone - chief of Cullentra
Ó hAiniarriadh - Henry - Southeast Ulster
Mac Einri - Henry - North Anthrim/Derry
Ó hOireachtaigh - (O) Heraghty - Co Galway and Mayo
Ó hEisleanáin or Ó hEislin - (O) Heslin - Co Leitrim - a sept of Breffny
Ó hIcidhe - Hickey - Co Clare & N. Tipperary - the Dal gCais
Ó hUigín - Higgins - Co Westmeath, Sligo and Mayo - Southern Uí Neill
Ó hÓgáin - Hogan - Co Clare & Tipperary - the Dal gCais
Ó hUallacháin - (O) Holohan - Co Offaly - of Clan Colgan with the O'Hennessy's
Ó hEoghanáin - (O) Honan - Co Clare and Co Cork
Ó hUaithnín - (O) Honeen, Hoyne, Greene - Co Clare and Tipperary
Ó hOdhráin - (O) Horan - Co Galway and Mayo
Ó hUallacháin - Houlihan - West Munster and Thomond
Mac Aodha or Aoidhe - MacHugh - West Galway - branch of O'Flahertys
Mac Aodha or Aoidhe - MacHugh - North Galway
Mac Aodha or Aoidhe - MacHugh - Co Fermanagh
Ó hAodha - Hughes - Ulster
Ó Urthaile - (O) Hurley - Co Clare & N. Tipperary - the Dal gCais
Ó hUirthile - (O) Hurley - West Cork
Ó hIarlatha - (O) Herlihy or (O)Hurley - Cork
Ó hEidhin - (O) Hynes or (O)Heyne -Galway - Uí Fiachrach Aidne
Mac Giolla Bháin - MacIlwaine or Kilbane - Co Sligo - the Uí Fiachrach
Mac Giolla Riabhaigh - MacIlrea, Kilrea - Co. Sligo & elsewhere
Mac Giolla Ruadh - MacIlroy, McElroy, MacGilroy, Kilroy - various
Mac Giolla Riabhaigh - MacIlwraith - Scottish
Mac an Airchinnigh - MacInerney - Co Clare - son of the erenagh
Ó Cathain - Kane or O'Cahan - East Donegal/Derry - Cenel Eoghain
Caomhanach - Kavanagh - Wexford and Carlow - descent from Donal MacMurrough
Ó Cadhla - (O) Kealy or Kelly - Co Galway - chiefs of Connemara
Ó Caollaidhe - (O) Kealy or Kelly - of Uí Buide, Co Laois
Ó Caollaidhe - (O) Kealy - of Uí Bercháin, Co Kilkenny
Ó Cathain - Keane - Co Galway (or Thomond) - Uí Fiachra Aidne
Ó Cein - Kean - Co Waterford
Ó Cearrnaigh - Kearney - Co Clare - Dalcassian
Mac Cearnaigh - Kearney - Ulster - Cenel Eoghain
Ó Catharnaigh - Kearney - Co Meath & Offaly - chiefs of Teffia
Ó Ciardha - (O) Keary - Co Kildare - the Southern Uí Neill, lords of Carbury
Ó Géibheannaigh - (O) Keaveney - Co Galway - the Uí Maine
Ó Caoimh or Caim - O'Keefe - Co Cork - descent from Art, King of Munster (742-762)
Ó Cianáin - (O) Keenan - Co Monaghan/Louth (Oriel)
Ó Caomháin - (O) Keevan or Kavanagh - Co Sligo - Uí Fiachrach
Ó Céileachair - Kelleher - Co Clare - the Dal gCais
Ó Ceallaigh - (O) Kelly - E Galway & S. Roscommon - Uí Maine (or Hy Many)
Ó Ceallaigh - (O) Kelly - Co Meath - O'Kellys of Breagh
Ó Ceallaigh - (O) Kelly - Co Laois - 7 Septs of Leix
Ó Caellaighe - (O) Kelly - Ossory, Kilkenny/Laois border
Ó Ceallaigh - (O) Kelly - Co Antrim and Down - Cinel Eachrach
Mac Cionath - MacKenna - Co Meath and Monaghan - southern Uí Neill
Ó Cinnéide - Kennedy - Co Clare/Tipperary - the Dal gCais - lords of Ormond
Ó Cionnaoith - (O) Kenny - Co Galway/Roscommon - Uí Maine
Ó Caoinnigh - (O) Kenny or Keeney - Co Leitrim/Donegal - lords of Muinter Kenny
Ó Coinne or O Coinnigh- Kenny or MacKenna - Co Down
Mac Eochaidh - Keogh - S. Roscommon - the Uí Mhaine, lords of Magh Finn
Mac Eochaidh - (Mac) Keogh - West Tipperary
Mac Eochaidh - Keogh - N. Kildare - same stock as the O'Byrnes
Mac Eoghain - MacKeon - Co Sligo/Leitrim
Ó Ciabháin - (O) Kevane or Kavanagh - Co Cork - Corcu Laoidhe
Ó Cadhla - Kiely - Co Clare/Limerick - the Dal gCais
Ó Céirín - (O) Kieran or Kearns - Co Mayo - chiefs of Costello
Ó Ciarain - Kerin or Carey - Co Cork/Clare
Mac Giolla Rua - Kilroy, Gilroy - Co. Clare & elsewhere
Ó Cinnfhaelidh - (O) Kinnealy - Co Limerick
Chennselaig or Cinnsealach - Kinsella - Co Wexford - see MacMurrough
Ó Ciardubháin - (O) Kirwan - Co Galway - one of the Tribes of Galway
Ó Leathlaobhair - (O) Lalor or Lawlor - Co Leix - 7 Septs of Leix
Ó Longáin - (O) Langan or Long - Co Armagh chiefs
Ó Lonagáin - (O) Lanigan - Co Tipperary chiefs
Ó Lorcáin - (O) Larkin - Co Wexford - chief of Fothart
Ó Lorcáin - (O) Larkin - Co Armagh - chief of Farney and West Uí Breasail
Ó Lorcáin - (O) Larkin - Co Galway - the Uí Maine
Ó Lorcáin - (O) Larkin - Co Tipperary - eranaghs of Lorrha
Mac Lochlainn - MacLaughlin - Co Donegal (Inishowen) - the Northern Uí Neill
Ó fLaithbheartaigh - (O) Laverty or Laherty - Co Donegal - lords of Aileach
Ó Limhain - (O) Lavin - Co Roscommon
Ó Laochdha - Leahy - Co Cork - Corca Laoighde
Ó Laoghaire - (O) Leary - Co Cork
Ó Léanacháin - Lenaghan, Linehan or Lenihan - Co Roscommon, Limerick/Tipperary
Ó Leannain - Lennon - Galway, Mayo and Fermanagh
Mac Giolla Mhín - MacLiveen - South Down
Ó Longargain - (O)Lonergan - Co Tipperary
Ó Longain - Long or (O)Langan - Co Armagh
Ó Longaigh - Long - Co Cork
Ó Lochlainn - (O)Loughlin or Lochlain - Co Clare - lord of Corcomroe
Ó Maoilsheachlainn - MacLoughlin or O Melaghlin - Co Meath - of Malachy II (southern Uí Neill)
Mac Lochlainn - MacLoughlin - Co Donegal - northern Uí Neill
Ó Labhradha - (O)Lowry or Lavery - Co Down and Ulster
Ó Lúinigh - (O)Lunney - Co Donegal/Tyrone - Cenel Moen
Ó Loideáin - Lydon, Leyden or (O)Liddane - Co Galway/Mayo
Ó Loinsigh - (O) Lynch - various including chiefs of the Dal Riada
Ó Loinsigh - Lynch - Co Galway - Norman origin, one of the Tribes of Galway
Ó Laighin - Lyons or Lyne - Co Galway and Kerry - Firbolg origin
Ó Liatháin - Lyons or Lehane - Co Cork and Limerick - Uí Fidhgheinte (also a Norman name)
Mac Maolruanaidh - Macarooney - Co Fermanagh
Ó Macdha - (O) Mackey - Co Tipperary - an Ormond sept
Ó Madaidhin or Matudain- Madden - East Galway - Uí Maine tribe
Mac or Mag Aodha - Magee - West and East Ulster (respectively)
Mag Uidhir - Maguire - Co Fermanagh - rulers of Fermanagh
Ó Meachair- Maher or Meagher - North Tipperary
Mac Mathghamha - MacMahon - Co Monaghan - rulers of Oriel (Airgialla)
Mac Mathghamha - MacMahon - Co Clare - the Dal gCais, descent from Mahon O'Brien
Ó Mathghamhna - (O) Mahony - Co Cork - Cenel Aodha, of the Eoghanacht
Ó Máille - O'Malley - Co Mayo - chiefs of Fir Umaille, or Hymalia
Ó Maoil Eoin - Malone - offshoot of the O'Connors of Connacht (?)
Ó Maoileoin - Malone - Co Clare and Wexford - assoc. with the Abbey of Clonmacnois
Ó Mainnín - Mannion or Manning - Co Galway - Pictish origin
Mac Maghnais - MacManus - Co Roscommon - from Maghnais, son of Turlough O'Connor
Mac Maghnais - MacManus - Co Fermanagh - from Maghnuis Maguire, son of Donn Mor
Ó Meadhra - O'Mara or O'Meara - Co Tipperary - chief of Uí Fahy
Mac Giolla Mhartain - Martin or Gilmartin - Co Tyrone - Uí Neill
Ó Miadhachain - Meehan or Meighan - offshoot of MacCarthys of Desmond
Ó Midhir - (O) Meere - Co Clare - Anglo origin
Ó Mealláin - (O) Mellan - Co Tyrone - Cenél Eóghain of Meallanacht
Ó Maoilédigh - (O) Melody - Co Clare/Galway - Corcomroe
MacMeanman -MacMenamin - Co Donegal/Tyrone - Cenél Conaill
Ó Muilleoir - Miller - Co Antrim - Scottish origin, Clan McFarlane
Ó Mochain - Mohan or Moen - Kilmacduagh, Galway and Killaraght, Sligo
Ó Maolmhuaidh - Molloy or Mulloy - Co Offaly - Southern Uí Neill
Ó Maolmhuaidh - (O) Molloy - Offaly - Southern Uí Neill, lords of Fercal
Ó Maoil Aodha - Molloy or Miley - Co Roscommon / East Galway
Ó Maol Dhómhnaigh - Moloney, Mullowney or Maloney - Co Clare - Chiefs of Kiltannon (Dal gCais)
Ó Manachain - Monaghan - East Roscommon - rulers of the 3 Tuathas
Ó Manachain - Monaghan - Co Fermanagh - the Fir Manach based in Lurg
Ó Maonaigh - Mooney - Co Offaly and various other
Ó Móráin - Moran - Co Mayo, near Ballina
Ó Mughráin - Moran - Galway/Roscommon - the Uí Maine, chief of Criffon
Ó Mórdha - (O) More or Moore - Co Laois - leading sept of the 7 Septs of Leix
Ó Murchain - Morgan - Co Managhan - mainly a name of Welsh origin
Ó Muircheartaigh - (O) Moriarty - South Kerry - same stock as O'Donoghues
Ó Muirgheasa - Morris - Co Sligo - the Uí Fiachrach
Ó Maolchathaigh - (O) Mulcahy - Co Tipperyary
Ó Maoldúin - (O) Muldoon - Co Fermanagh - chiefs of Lurg
Ó Maoldúin - (O) Muldoon - Co Clare (Thomond) - the Uí Bloid
Ó Maoldúin - (O) Muldoon - Co Sligo - Uí Fiachrach
Ó‘Maoldoraidh - O'Muldory - Donegal - Tirconnel (MacAlinions)
Ó Maoldomhnaigh - Muldowney - Co Kilkenny
Ó Maolchathail - (O) Mulhall or Mulcahill - Co Laois - descendant of the devotee of (St.) Cathal
Ó Maolchalann - Mulholland - N. Limerick - descendant of the devotee of (St.) Calann
Maoilchiaráin - Mulkerrin, Mulkieran - Roscommon, Galway, Ulster - devotee of (St.) Ciaran
Ó Maolalaidhs - O'Mullally or Lally - Co Roscommon - the Uí Maine
Ó Maolain - Mullan, Mullin(s), Mullen, Mullane - Co Galway - descent from Maolan
Ó Maolain - Mullan, Mullin(s), Mullen, Mullane - Co Derry - followers of the O'Cahans
Ó Maoilédigh - (O) Mulledy - Co Westmeath/Offaly - Ballymaledy
Ó Maolain - Mullins and Mollins - Cork and Monaghan (respectively)
Ó Maolagáin - (O) Mulligan - Co Donegal - chiefs of Tir MacCarthain
Ó Maolchaoine - (O) Mulqueen - Co Clare, Limerick - Dalcassian, from St. Caoine
Ó Maolruanaidh - Mulrooney - Co Fermanagh and in Mayo/Roscommon
Ó Maoilriaghain or Mael Riain- O'Mulryan or Ryan - Co Limerick/Tipperary
Ó Murnain - Murnane - Murnane - Co Limerick
Mac Murchada - MacMurrough - Co Wexford (Hy Chennselaig) - descendant of Murchadh
Ó Murchadha and Mac Murchadha - Murphy - Co Tyrone - the Cenel Eoghain
Ó Murchadha - Murphy - Co Wexford (Hy Felimy)- descendant of Murchadh or Murrough
Ó Muireachaidh - Murray - Roscommon/Galway - the Uí Maine
Ó Muireadhaigh - Murray - West Ireland - including a sept of the Uí Maine
Mac Muireachaidh - (Mac)Murry or MacMorrow - Co Leitrim
Ó Muircheartaigh - Murtagh or MacMurtry - Co Meath
Mac an Fhailghigh - Nally or MacNally - Co Roscommon and Mayo - Norman origin
Mac Con Uladh - MacNally or MacAnally - Co Armagh and Monaghan (S.E. Ulaid)
Mac Conmara - MacNamara - Co Clare - the Dal gCais, rulers of Clancullen
Mac Conmidhe - MacNamee or Conway - Co Derry and Tyrone
Ó Neachtain - (O) Naghten - Co Clare - Dalcassian
Ó Neachtain - (O) Naughton or Naghten - Co Roscommon/Galway - the Uí Maine
Ó Neill- O'Neill - Co Tyrone/Derry/Donegal - the Cenel Eoghain, rulers of Tir Eoghain
Mac an Fhilidh - (Mac) Neilly, Neely - Co Antrim
Mac Aonghasa - MacNiece - Co Antrim
Ó Niallain - Neilan or Neylan - Co Clare (Thomond)
Mac an Airchinnigh - Nerney - Co Roscommon - son of the erenagh
Ó Nuallain - Nolan - Co Carlow - princes of Foharta (Fothairt)
Ó hIonmhaineáin - (O) Noonan - Co Cork
Mac Phaidin - Patterson - Ulster (and Scotland)
Ó Peatáin - Peyton or Patton - Co Donegal - Cenél Moen
Ó Faolain- Phelan or Whelan - Co Waterford - rulers of the Decies
Ó Coigligh - Quigley - Co Mayo - the Uí Fiachrach
Ó Cuill - (O) Quill - Co Cork and Kerry
Mac an Choilgh - MacQuilly or Cox - Co Roscommon
Ó Caoindealbháin - (O) Quinlan or Kindellan - Co Meath - southern Uí Neill
Ó Cuinn - (O) Quinn - Co Tyrone - Cenel Eoghain
Ó Cuinn - (O) Quinn - Co Clare (Thomond) - the Dal gCais
Ó Cuinn - (O) Quinn - Co Antrim - the Glens of Antrim
Ó Cuinn - (O) Quinn - Co Longford - same stock as O'Ferralls of Annaly
Ó Cuirc - (O) Quirke - Co Tipperary - chiefs of Muscraighe Cuirc
Ó Raithbheartiagh - (O) Rafferty - Co Donegal/Sligo
Mac Raghnaill - MacRannall or Reynolds - Co Leitrim - lords of Muintir Eolais
Ó Reachtagáin - (O) Ratigan - Co Roscommon - coarbs of St. Finnen
Ó Riordan - Rearden or (O)Riordan - Co Tipperary/Cork - followers of the Lords of Muskerry
Ó Rodáin - (O) Reddan - Co Clare - stewards to O'Brien and MacNamara
Ó Riagáin - (O) Regan - Co Meath/Dublin - the Four Tribes of Tara
Ó Raogáin - (O) Regan or Reagan - Co Limerick - the Dal gCais
Ó Riada - (O) Reidy - Munster - king of Aradh in the 12th century
Ó Raghallaigh - (O) Reilly - Co Cavan - Uí Briuin Breifne
Ó Maolcraoibhe - Rice - Co Louth/Armagh - also of Welsh origin
Ó Ruanaidh or O Mael Ruanaid - Rooney - Co Down
Ó Roghan - (O) Rogan - Co Down - of Armagh and Iveagh
Mac Ruaidhri - MacRory or MacCrory - Co Tyrone
Ó Ruairc - (O) Rourke - Co Leitrim/Cavan - the Uí Briuin - rulers of Breifne
Ó Ruaidhín - (O) Ruane or Rowen - Co Mayo - the Uí Fiachrach
Ó Ruadháin - (O) Ruane or Rowen - Co Galway - the Uí Maine
Ó Riaghain - Ryan - Co Carlow - chiefs of Uí Drona, also see Mulryan
Ó Scannláin - (O) Scanlan - Munster
Mac Scannláin - (Mac) Scanlan - Louth (Oriel)
Ó Scolaidhe - (O) Scully - Co Westmeath
Ó Scolláin - Scullion or (O) Scully - Co Derry
Ó Seasnáin - Sexton - Thomond (also an English name)
Ó Seanachain- (O) Shanahan - Co Clare - Uí Bloid (Dal gCais)
Mac Seanlaoich - (Mac) Shanly - Co Leitrim - MacShanly of Dromod
Ó Seanáin - (O) Shannon - Carlow/Wexford
Mac Giolla t-Seanáin - (O) Shannon - Co Clare - son of the follower of St. Senan
Mac Searraigh - MacSharry or Foley - Breifne, e.g. northern Leitrim, Sligo
Ó Seachnasaigh - (O) Shaughnessy - Kiltartan, Co Galway - the Uí Fiachra Aidhne
Ó Seaghdha or Segda - (O) Shea - South and East Kerry - Lords of Iveragh
Ó Siodhacháin - Sheehan - Co Limerick/Cork - the Dal gCais
Ó Sirideáin - Sheridan - Co Longford/Cavan
Ó Seiridh or Mac Seiridh - O'Sherry or McSherry - Oriel, e.g. Monaghan and Armagh
Mac an Ghirr - Short, Shortman or Shortel - Unknown
Ó Sluaghain or O’Sluagahadain - Sloan - of Scottish origin
Mac Gabhann - Smith or MacGowan - Co Cavan
Ó Sóchlacháin - (O) Solan, Solahan - Co Mayo
Ó Spealáin - (O)Spillane - Co Tipperary, then Cork & Kerry
Ó Spealáin - Spillane or Spelman - Co Sligo
Ó Suileabháin - (O) Sullivan - Co Kerry - the Eoghanacht - O'Sullivan Mor of Kenmare Bay
Ó Suileabháin - (O) Sullivan - Co Cork - Eoghanacht - O'Sullivan Beare of Beara Peninsula
Mac Suibhne - Sweeney or MacSweeny - Co Donegal - descent of Scottish chief in Argyle
Ó Tarpaigh - (O) Tarpy, Tarpey, Torpey - Co Sligo - lords of Carra
Mac Tighearnáin - (Mac) Tiernan or Kiernan - Co Cavan - chiefs of Teallach Donnchadha
Mac Tighearnáin - (Mac) Tiernan - Co Roscommon - descent of O'Connor
Ó Tighearnaigh - (O) Tierney - Co Mayo - lords of Carra
Ó Tighearnaigh - (O) Tierney - Co Galway, Limerick and Tipperary - Ormond's Fir O'Tyernie
Ó Tuathail - O'Toole - Co Kildare and Wicklow - descent from Tuathal, king of Laigin
Ó Treasaigh - (O) Tracy or Treacy - Co Cork, Galway, Leix (Lords of Slievemargy)
Ó or Mac Tréinfhir - Traynor - Co Meath
Ó Tiobraide - (O) Tubridy - Co Clare
Ó Tuama - O'Twomey or Toomey - Co Cork, and Co Limerick
Mac an Bháird - (Mac) Ward - Co Donegal and Galway
Ó Faolain - Whelan or Phelan - Co Waterford - rulers of the Decies