Alphabetical List of Irish Names: O
Irish Surnames | A-B | C | D | E-G | H-J | K-L | Mac | M-N | O | P-R | S-W
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| Name | Variants | In Irish | Location | Origin | Comments |
|---|---|---|---|---|---|
| O'Brien | Clare, Limerick, very widespread. | Gaelic | Descendents of Brian Boru, High King of Ireland. | ||
| O'Callaghan | Callaghan, Callahan | Ó Ceallacháin | Cork, Limerick, widespread. | Gaelic | Probably originated from Ceallachan, a 10th century King of Munster. |
| O'Casey | Casey | O Cathasaigh | Gaelic | Means "vigilant". | |
| O'Connell | Ó Conaill | Derry, Galway, Kerry widespread, | Gaelic | Very old name. | |
| O'Connor | O'Conor, Conner, Connor | Ó Conchobhair, | Clare, Cork, Kerry, Offaly, Roscommon, widespread. | Gaelic | Means "hero or champion"; descendents of King Conchobhair of Connacht. . |
| O'Dea | O Deaghaidh | Clar, Cork, Dublin, Limerick, Tipperary | Gaelic | ||
| O'Donnell | Ó Domhnaill | Donegal. | Gaelic | Means "world mighty" | |
| O'Donovan | Ó Donnabhain | Cork, Kilkenny | Gaelic | Two colours in the name: donn meaning "brown" and dubhann "black". | |
| O'Dowd | Dodd, Doody, Duddy | O Dubhda | Galway, Mayo, Sligo | Gaelic | Means "black". |
| O'Driscoll | Driscoll. | O hEidersceoil | Cork | Gaelic | Means "go between or interpreter". |
| O'Dwyer | Dwyer | O Dubhuidir, | Mayo, Sligo. | Gaelic | Means "black shirt". |
| O'Flaherty | Flaherty, O'Flaverty. O'Laverty | Ó Flaithbheartaigh | Galway, Mayo | Gaelic | Very ancient family |
| O'Gara | O Gadhra | Cavan, Mayo, Sligo. | Gaelic | Gadhra means "a mastiff " or "dog" | |
| O'Gorman | Gorman, Grimes | O Gormain | Clare, Laois, Monaghan. | Gaelic | |
| O'Grady | Grady | Ó Grádaigh | Clare, Galway, Limerick. | Gaelic | Means "illustrious"; part of the Dalcassian sept and so related to the O'Briens. |
| O'Hagan | Hagan, Aiken | O hAodhagain. Ó hAgain, | Tyrone, Ulster | Gaelic | Means "young". |
| O'Halloran | Halloran | O hAllmhurain | Clare, Galway. | Gaelic | Means "stranger from over the sea". |
| O'Hara | O hEaghra | Antrim, Sligo | Gaelic | ||
| O'Hegarty | Hegerty, Hagerty | O hEigceartaigh | Cork, Derry, Donegal. | Gaelic | Means "unjust" |
| O'Higgins | O hUigin | Sligo, Clare, Galway, widespread. | Gaelic | Means "knowledge or ingenuity". | |
| O'Keeffe | O'Keefe | Ó Caoinmh | Cork, Limerick | Gaelic | Caom means "noble" or "gentle". Also means "descendent of Caoinmh", himeself son of Fionghuine, a King of Munster. |
| O'Leary | Leary | O Laoghaire | Cork | Gaelic | Means "calf keeper" |
| O'Malley | Melia | Ó Máille | Mayo | Gaelic | Old and distinguished family. The name is derived from "maglios" an ancient Celtic word meaning "chief". |
| O'Meara | Meara, O'Mara, Mara | O Meadhra, | Tipperary, widespread. | Gaelic | Means "mirth". |
| O'Neill | Neill, Nihill | Antrim, Down, Tyrone, widespread. | Gaelic | Very old and prestigious family. Neill means "champion". | |
| O'Reilly | Reilly, Riley, O'Rahilly | Ó Raghailligh | Cavan, widespread | Gaelic | Ragheallach means "gregarious race" |
| O'Riordan | Riordan, Reardon. | O Riordáin | Cork, Tipperary. | Gaelic | Means "royal bard". |
| O'Rourke | Roark, Rooke, Rourke, Rorke | Ó Ruairc | Cavan, Leitrim. | Norse | Originates from the Norse-Viking name Hrothrekr, who married into the Irish. |
| O'Shaughnessy | Shaughnessy | O Seachnsaigh | Clare, Galway, Limerick. | Gaelic | |
| O'Shea | Shea, Shee | Ó Séaghdha, Ó Sé | Kerry, Kilkenny. | Gaelic | Means "stately or majestic" and also "dauntless". One of the merchant families of Kilkenny. |
| O'Sullivan | Sullivan | Ó Súileabháin | Cork, Kerry, Tipperary. | Gaelic | Means "one-eyed" or "hawk-eyed". |
| O'Toole | Toal, Tool, Toole, Toohill, Twohill. | Ó Tuathail | Kildare, Wicklow, widespread. | Gaelic | Descendents of Tuathal, a 10th century King of Leinster. Means "mighty or prosperous people" |
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