Mc, Mac = son of 
O = can signify either grandson, or earlier ancestor 
Fitz, from French fils = son of 
Ní, Nic = girl's use before her father's name 
Ban = wife's use before her married name 

The Counties and places listed are where the names are/were (most) prominent prior to 1700. 
A
Ahearne — From Ó hEachtighearna, meaning "lord of the horse". 

B
Barrett — Báróid. Norman origins. Cork, Galway, Mayo. 

Barry — from Wales.  Barrymore. Cork. 

Beirne — Ó Beirne.  Roscommon.

Blake — Norman origins, Galway.

Boland — Ó Beolláin. Cork, Derry. 

Brady — MacBrádaigh, "spirited".  Cavan, Leinster, Ulster. 
Breen — O Braoin, "sadness, sorrow". MacBreen, Brawney. Kilkenny, Westmeath, Wexford. 

Brennan — O Braonáin, meaning "little drop" or "sorrow".  Kerry, Kilkenny, Westmeath. 

Brody — MacBruaideadha. Clare. 

Brown — Norman origins. Galway.

Buckley — O Buachalla. Cork, Kerry. 

Burke — Norman origins. Widespread. 

Butler — Norman Origins. Widespread.

Byrne — see O'Byrne. 

C
Cahill — Originally MacCathail, meaning "manly" or "valour".  Clare, Galway, Tipperary. 

Carey — O Ciardha. O'Kerry, Kerry, Carew. Kerry, Kildare. 

Carroll — Ó Cearbhaill, from Cearbhal, "warlike champion".

Cassidy — O Caiside. Fermanagh originally, now widespread. 

Clancy — MacFhlannchaidh, "ruddy warrior". Clanchy, Glanchy, MacClancy. Clare, Leitrim. 

Clarke — Cleireach, "clerk". Clerys changed their name to Clarke after Gaelic Irish names were outlawed by English conquerors. O'Cleirigh, O'Clery, Macalary, MacClery. Ulster. 

Clery — Ó Cleirigh; cléireach is the Irish word for "clerk." Cleary, Clarke. 

Coghlan — MacCochlain or O Cochlain, "cape or hood". Coughlin, O'Coughlan, Coughlan, Cohalan, MacCoughlan. Cork, Offaly. 
Collins — anglicized from the Irish Ó Cioleáin, meaing "young whelp or young creature".  Cork. 

Conaghan — Ó Connacháin, usually found anglicized as Cunningham. Ulster. 

Connolly — Ó Conghaile, the original Gaelic Conghaile means "valorous".  Connacht, Cork, Monaghan, Ulster. 

Conroy — O Conratha, "hound of prosperity". O'Mulconry, Mulconry, Conary, Conree, Conry. Clare, Roscommon. 

Conway — MacConnmhaigh. Clare, Galway, West Cork. 

Corcoran — O Corcrain, meaning "ruddy".  Fermanagh, Kerry, Mayo, Offaly. 

Costello — MacOisdealbh, "son of Oistealb". Connacht. 

Costelloe — A a separate clan from Costello. Name place?

Crowley — O Cruadhlaoich, "hunchbacked" or "strong hero". Cork, Roscommon. 

Cuif — French version of the surname O'Keeffe. O'Keefe. 

Cullen — Ó Cuillin, "holly tree". Founders of Kilcullen in County Kildare. Possibly connected with Scottish clans.  Kildare, Wexford. 

Cummins — O Comáin, "a hurley".  Kerry, Limerick, Mayo. 

Curran — O Corráin, "a descendant of O'Corrain". Galway, Kerry, Ulster, Waterford. 

Curtin — Mac Cuirtin, at one time in Irish it was Cruitín "hunchback".  Cork, Dublin, Limerick. 

Cusack — Cíomhsóg. Known in County Mayo as MacIosóg. 

D
Daly — Ó Dálaigh. Dawley. 

Delany — O Dubhshlaine, "of the river Slaney". From dubh "black" + Slaine "the river Slaney". Delane, Delaney. Dublin, Leix. 

Dempsey — O Diomasaigh, "proud". Leix, Offaly. 

Devine — O Daimhin, "poet, bard". Davin, Devane, Devin, Downes. Cavan, Dublin, Fermanagh, Louth, Tyrone. 

Devlin — O Doibhlin. Sligo, Tyrone. 

Dillon — From Brittany.

Doherty — Ó Dochartaigh, meaning "obstructive". Ulster. 

Dolan — O Dobhailen, "black defiance, challenge". Doolan, Dowling, Doelan, O'Doelan. Galway, Roscommon. 

Donoghue — Ó Donnchadha, comes from the personal name Donnchadh (Donogh).  County Kilkenny.

Doran — O Deoradháin, "exile or stranger". Armagh, Down, Kerry, Leix, Wexford. 

Dowling — O Dunlaing. Carlow, Dublin, Kilkenney, Leix, Wicklow. 

Doyle — Ó Dubhghaill, from dubh-ghall "dark foreigner." Wexford. 

Driscoll — see O'Driscoll. 

Duffy — Ó Dubthaigh, Roscommon, also Ulster. 

Duggan — O Dubhagain, "black head". Doogan, Dougan. Mostly Cork, also Donegal, Galway, Tipperary. 

Dunne — Ó Duinn, "brown". 

E
Egan — Mac Aodhagáin, "son of Aodh", anglicized to Hugh. Their origins are in Galway, Roscommon and Leitrim. 

F
Fagan — O Faodhagain, "little Hugh". O'Hagan, Fegan, O'Hogan. Dublin, Kerry. 

Fahy — O Fathaigh, "field green". Fahey, Faghy, Green. Galway, Tipperary. 

Fallon — O Fallamhain, "ruler". Falloon. Cork, Donegal, Galway, Kerry Wexford. 

Farin — Mac Fearachàin, Mac Mhearain, possible form of Ferran. 

FitzGerald — fitz means "son" of Norman orgins.

Fitzpatrick — MacGiolla Padraig, "servant of St. Patrick". The only Fitz surname that is Irish and not Norman in origin. Leix, widespread. 

Flanagan — O Flannagain, "red". O'Flannagan. Fermanagh, Offaly, Roscommon. 

Flynn — O Floinn, "red, ruddy". Antrim, Cork, Roscommon. 

Fogarty — cognate with Gogarty; O Fogartaigh, "exiled, banished". Tipperary. 

Foley — O Foghladha, "plunderer". Mostly Munster, Waterford. 

Friel — Ó Frighil, "man of valor"; form of Fearghal (Farrell). 

G
Gaffney — O Gamhna, "calf." Connacht. 

Gallagher — Ó Gallchobhair. 

Galvin — (See Gallivan below)

Gallivan — Ó Gealbháin, "bright white". Kerry, Roscommon. 

Garvey — O Gairbith or Mac Gairbhith, "rough peace". Armagh, Donegal, Down, Kilkenny. 

Geraghty — Mag Oireachtaigh, Garrity, Gerity, Galway, Roscommon. 

Gilday — Mac Giolla Dhé, meaning "the son of the devotee of God." Originally from County Tirconnell and now found in Connacht and Clare. Gildea. 

Griffin — Ó Grifín and the Ó Gríobhtha/Ó Gríofa. Clare and Kerry.

Guinness — MagAonghusa, "son of Aonghus." Ulster. 

H
Hagan — see O'Hagan. 

Healy — Ó hÉildhe. Cork, Limerick, Tipperary. 

Hickey — O hIcidhe, "healer". Clare, Limerick, Tipperary. 

Higgins — Ó hUigin. 

Hogan — Ó hOgáin. Clare, Limerick, Thomond, Tipperary. 

J
Jennings — MacSheóinín. Connacht. 

Jorden — An anglicization of the surname Mac Siurtáin. 

Joyce — From Seoigh. Connacht, Galway, Ross. 

K
Kavanagh — Caomhánach, from Diarmuid MacMurrough, 12th C. King of Leinster. Cavanagh, MacMurrough. 

Keane — Ó Cathain, Mac Cathain of West Clare, Ó Cahain of Ulster and Ó Céin of Munster. Munster, Ulster, West Clare. 

Kelleher — O Céileachair, "loving spouse". Keller. Clare, Cork, Kerry. 

Kelly — see O'Kelly. 

Kennedy — see O'Kennedy. 

Keogh — Anglicized from the Irish name MacEochaidh. Leinster, Wexford. 

Killoran — From Mac Giolla Luairinn and Mac Giolla Luaithrinn (also Luaran/Loran).  Sligo. 

Kinsella — O Cinnsealach, from Mac Murrough. Wexford, Wicklow. 

Kirwan — Ó Ciardubháin, meaning "black." Louth, Galway. 

L
Lacy — de Léis, From Lascy in Normandy. 

Lalor — O Leathlobhair, Gaelic for "half leper". Lawlor. Leix. 

Lee — O Laoidhigh. Cork, Galway, Limerick, Tipperary. 

Lennon — Ó Leannáin. Mayo and Galway. 

Lynch — Ó Loingsigh,  Clare, Sligo and Limerick, and Donegal. Also Cork.

Lyons — O Laighin or Ó Liathain.  Galway. 

M
MacAuley — MacAmhlaoibh.  Cork, Fermanagh, Westmeath. 

MacAuliffe — MacAmlaoibh, of Nors origin. Cork. McAuliffe. 

MacBride — MacGiolla Brighde, meaning "devotee of St. Brigid". Donegal, Ulster. McBride. 

MacCabe — Mac Cába. Originally from Scottland. Ulster.

MacCann — MacAnnadh, from Armagh of Ulster. 

MacCarthy — MacCarthaigh, "loving one". Roscommon, Sligo. 

McElrath — Mac Giolla Riabhaigh, also MacIlrea, Kilrea (County Sligo) MacIlwraith, MacIlrath from Ulster and Scotland. 

MacElroy — MacGiolla Rua. Fermanagh, Leitrim. McElroy. 

MacEvoy — MacGuiollabhuidhe, mean "the woodsman." Leix, Louth. 

MacFrederick —  Northern Ireland via Scotland. 

MacGee —  Antrim, Armagh, Down. 

MacGill —  From Mac An Ghaill or "son of a foreigner". Eastern Ulster. 

MacGovern — Mag Samhrain, meaning "summer." Cavan, Fermanagh, Leitrim. 

MacGrath — MacRaith or Mag Raith. Clare, Donegal, Fermanagh, Limerick. 

MacHugh — From MacAoda, "son of Hugh". Septs and varients: Hewson, MacCoy, McKee, Hughes, MacKay, McHugh. Donegal, Fermanagh, Galway, Leitrim, Mayo. 

MacInerney — Mac An Airchinnigh, "steward of church lands." Connacht.

MacKenna — Mac Cionaoith, Irish MacCionaoda "son of Cionaoid". Ulster. 

MacLoughlin — O Maoilsheachlain, "follower of St. Secundinas." Connacht, Fermanagh. 

MacNally — Mac an Fhailghigh, meaning "poor man". Armagh, Mayo, Monaghan. 

MacNamara — "son of the hound of the sea." There were two septs both from Clare. 

MacNulty — Mac An Ultaigh, "ulidian", "of Ulster". McNulty, Nulty. Ulster, Mayo. 

MacQuaid — Mac Uaid, "son of Walter."  Ulster. 

MacQuillan — MacCoilin or MacUighlilin.  Ulster. 

MacSweeney — Descendants of Suibhne O'Neill, a chieftain of Scotland. Cork, Donegal. 

McKowen — MacEoghain. Connacht.

McGinn — From the Mag Fhinn surnames. Counties Tyrone and Down.

Madden — O Madáin, meaning "small dog". Galway, Limerick, Longford, Offaly. 

Maguire — MagUidhir meaning "pale colored". Fermanagh

Maher — O Meachair, "hospitable one". Meagher. Kilkenny, Tipperary. 

Mahoney — (see O'Mahony). 

Mallaghan — may be translated in two ways : "the gentle chief" or "the small bald man". Mallaghans were supposed to have been the wise men and advisers to the clan. 

Malone — Maoileoin "one who served St. John". Of Norman Decent

Molloy — O Maolmhudaidh meaning"the noble chief". Widespread throughout Ireland. 

Moloney — O Maoldhomhnaigh. Clare, Tipperary. 

Monaghan — O Manacháin, "the monk". Roscommon. 

Mooney — O Maonaigh. Offaly, Sligo, Ulster. 

Moore — Ó Mórdha, meaning the "noble."  Leix. 

Moran — O Móráin, "big, great" or "law giver." Connacht. 

Moriarty — O Muircheartaigh, "great navigator." Kerry. 

Morrissey — O Muirgheasa. Common in Cork, Limerick, Sligo, Waterford. 

Mulcahy — O Maol Chathaigh, "battle chief". Munster. 

Mulrooney — Maelranaidh, Ancestor from the MacDermot clan. 

Mulligan — Ó Maolagáin. Donegal, Mayo, Monaghan. 

Murphy — Ó Morchoe, "sea warrior".  Leinster, Sligo, Tyrone, Wexford. 

Murray — Ó Muireadhaigh; anglisiced to Murray in the 17th century. 

N
Nolan — O Nuaillain. West Cork. 

Nugent — Nuinseann.  Westmeath. 

O
O'Brien — take their name from the 10th C. Brian Boru, High King of Ireland. Clare, Limerick, Munster. 

O'Byrne — Ó Broin, Byrne. Wicklow. 

O'Callaghan — Ó Ceallacháin, meaning "strife". Kilkenny, Munster. 

O'Casey — O Cathasaigh, "the watchful". Munster. 

O'Connell — Ó Conaill.  Derry, Galway, Munster. 

O'Connor — Ó Conchobhair, "hero, champion." Clare, Munster, Offaly, Roscommon, Ulster. 

O'Dea — O Deaghaidh. Cork, Dublin, Limerick, Tipperary. 

O'Donnell — Ó Domhnaill, "mighty."  Donegal. 

O'Donovan — Ó Donnabhain. Cork, Kilkenny. 

O'Dowd — O Dubhda. Galway, Mayo, Sligo. 

O'Driscoll — O hEidersceoil.  Cork. 

O'Dwyer — O Dubhuidir, "black shirt". Mayo, Sligo. 

O'Farrell — Ó Fearghaill, "of great valour." Widespread.

O'Flaherty — Ó Flaithbheartaigh. Connacht. 

O'Gara — O Gadhra, "a mastiff " (dog). Mayo, Sligo. 

O'Gorman — O Gormain. Clare, Leix, Monaghan. 

O'Grady — Ó Grádaigh. Limerick. 

O'Hagan — O hAodhagain or Ó hAgain. Clare, Galway. 

O'Hara — O hEaghra. Antrim, Sligo. 

O'Hegarty — O hEigceartaigh, meaning "unjust." Cork, Derry, Donegal. 

O'Higgins — O hUigin. Leinster, Munster, Sligo. 

O'Keeffe — Ó Caoinmh. Galway, Roscommon. 

O'Kennedy — Ó Cinneide.  Clare, Kilkenny, Tipperary. 

O'Leary — O Laoghaire.  Cork. 

O'Mahony — From Mathghamhan. Cork, Munster. 

O'Malley — Ó Máille. Melia, Mayo. 

O'Meara — O Meadhra, meaning"mirth". Tipperary. 

O'Neill —  Niall means "champion". Creagh, Neill, Nihill. Antrim, Down, Tyrone, Ulster. 

O'Reilly — Ó Raghailligh. Cavan. 

O'Riordan — O Riordáin. Cork, Tipperary. 

O'Rourke — Ó Ruairc. Cavan, Leitrim. 

O'Shaughnessy — O Seachnsaigh. Shaughnessy, Sandys. Clare, Galway, Limerick. 

O'Shea — O Seaghada. Kerry, Kilkenny. 

O'Sullivan — Ó Súileabháin. Connacht, Kildare, Ulster, Wicklow. 

P
Phelan — O Faoláin or O hAoileain, meaning "joyful." Carlow, Kilkenny, Waterford, Wexford. 

Plunkett — French origins. Louth, Meath. 

Power — Norman origins.  Waterford, Wicklow. 

Q
Quigley — O Coigligh, Cogley, Coigley. Derry, Donegal, Galway, Mayo, Sligo. 

Quinlan — O Caoindealbháin. Kindellan, Quinlevan. Munster. 

Quinn — O Cuinn, meaning "intelligent". Quin. Antrim, Clare, Longford. 

R
Rafferty — O Reachtaire, or O Raithbheartaigh, O Robhartaigh, "flood tide". Connacht. 

Redmond — Réamonn, of Norman origins. Wexford, Wicklow. 

Regan — Ó Reagáin or Ó Riagain, Widespread.

Rice — Rhys. Dublin, Kerry, Louth, Ulster. 

Riggs — Very little written about this name. Widespread. 

Roche — de Róiste, of Norman origins.

Rooney — O Ruanaidh, meaning "hero". Down, widespread. 

Ryan — Ó Riain or Ó Maoilriain. Carlow, Leinster, Limerick, Tipperary. 

S
Scanlan — O Scannláin or MacScannlain. Scanlon. Connacht, Cork, Kerry, Limerick. 

Scully — O Scolaidhe.. Leinster, Tipperary. 

Shannon — O Seanacháin, "old, wise". Clare, Ulster. 

Sheehan — O Siodhacáin, "peaceful". Sheahan.Munster. 

Sheehy — Mas Sithigh. Munster. 

Sheridan — Ó Sirideáin. Little known about this name. Cavan. 

T
Taaffe — family came from Wales in 1196 to settle in Louth, and David is the Welsh equivalent. 

Táilliúir — (TAWLL-yur) Gaelic version of Taylor, brought by the English to Dublin and Ulster. 

Tierney — O Tighearnaigh. Donegal, Mayo, Tipperary. 

Tobin — Tóibín. Brittany, Munster. 

Treacy — O Treasigh, meaning "fighter". Cork, Galway, Leix. 

Tuite — Norman origins. Meath, Westmeath, Louth Longford. 

Tully — O Taicligh or Mac an Tuile, meaning "flood." Cavan, Connacht, Longford, Westmeath. 

Twomey — O Tuama. Toomey. Clare, Cork, Kerry, Limerick. 

W
Wall, de Valle — du Val, Norman origins. Carlow, Cork, Kilkenny, Limerick, Waterford. 

Walsh — Breathnach, Welsh origins. Dublin, Kilkenny, Leix, Waterford, Wicklow. 

Ward — Mac an Bháird, "bard or singer". Connacht, Ulster. 

Woulfe — French Norman origins. Cork, Kildare, Limerick.