| Families |
Ethnic Group |
Branch of |
| O'Connor |
IVL - Oirghialla |
|
| O'Connor |
Laigin |
|
| O'Corahy |
|
MacCoghlan |
| O'Cornyn |
Erainn |
|
| O'Corrigan |
Laigin |
|
| O'Crean |
Gaels |
|
| O'Crowley |
|
MacDermot |
| O'Cullane |
Erainn |
|
| O'Cullen |
Laigin |
|
| O'Cullinan, Cullen |
Gaels |
|
| O'Cullinan |
IVL - Cork |
|
| O'Cullinane |
IVL - Cork |
|
| O'Daly |
Gaels |
|
| O'Davoren |
Erainn |
|
| O'Dea |
Erainn |
|
| O'Dempsey |
Laigin |
|
| O'Devany |
IVL - West Ulster |
|
| O'Dinneen, Denning |
Erainn |
|
| O'Doherty |
Gaels |
|
| O'Donegan |
IVL - Tipperary |
|
| O'Donnell |
Gaels |
|
| O'Donnell |
IVL - West Munster, Galway |
|
| O'Donnellan |
IVL - Tyrone, Offaly |
|
| O'Donnellan |
Laigin |
|
| O'Donnelly |
Gaels |
|
| O'Donnelly |
IVL - Galway, Cork |
|
| O'Donoghue |
Gaels |
|
| O'Donohoe |
IVL - Galway, Cavan |
|
| O'Donovan |
Erainn |
|
| O'Donovan |
IVL - Cork, Tipperary |
|
| O'Doran |
Cruithne |
|
| O'Dowd |
Gaels |
|
| O'Dowd |
IVL - Derry |
|
| O'Dowling |
Cruithne |
|
| O'Dowling |
IVL - Cork, Leinster |
|
| O'Doyle |
Vikings |
|
| O'Driscoll |
Erainn |
|
| O'Duany, Devany |
|
O'Gara |
| O'Duggan |
IVL - Cork |
|
| O'Duigenan, Deignan |
Gaels |
|
| O'Dunne |
Laigin |
|
| O'Durack |
Erainn |
|
| O'Dwyer |
Erainn |
|
| O'Fahy |
Laigin |
|
| O'Fallon |
IVL - Offaly |
|
| O'Fallon |
Laigin |
|
| O'Farrell |
Erainn |
|
| O'Finnegan |
Gaels |
|
|
