| Families |
Ethnic Group |
Branch of |
| O'Finnegan |
IVL - Cavan |
|
| O'Flaherty |
Gaels |
|
| O'Flanagan |
Gaels |
|
| O'Flanagan |
Laigin |
O'Boylan |
| O'Flanagan |
Laigin |
O'Carroll |
| O'Flynn |
Erainn |
|
| O'Flyn |
IVL - Galway, Cork |
|
| O'Fogarty |
Erainn |
|
| O'Friel |
Gaels |
|
| O'Gallagher |
Gaels |
|
| O'Galvin |
Erainn |
|
| O'Gara |
Laigin |
|
| O'Garriga |
|
MacGarry |
| O'Gormley |
Gaels |
|
| O'Gormley |
IVL - Connacht |
|
| O'Grady |
Erainn |
|
| O'Grady |
IVL - Connacht |
|
| O'Griffey |
Erainn |
|
| O'Hagan |
Gaels |
|
| O'Hagan |
IVL - South Ulster |
|
| O'Halloran |
IVL - Clare |
|
| O'Halloran |
Vikings |
|
| O'Hanlon |
Laigin |
|
| O'Hanley |
Laigin |
|
| O'Hanraghty |
Laigin |
|
| O'Hanrahan |
Erainn |
|
| O'Hara |
Laigin |
|
| O'Hart |
Gaels |
|
| O'Hartagan |
Erainn |
|
| O'Hea |
Erainn |
|
| O'Heffernan |
Erainn |
|
| O'Hegarty |
Gaels |
|
| O'Hegarty |
IVL - Tipperary |
|
| O'Hennessy |
Erainn |
|
| O'Hennessy |
Laigin |
|
| O'Heyne |
Gaels |
|
| O'Hickey |
Erainn |
|
| O'Higgin, Higgins |
Gaels |
|
| O'Hoey |
IVL - Meath |
|
| O'Hoey |
|
MacDonlevy |
| O'Holohan |
Laigin |
O'Hennessy |
| O'Horan |
Laigin |
|
| O'Houlihan |
Gaels |
|
| O'Kearney |
|
Fox |
| O'Kearney |
Gaels |
|
| O'Keeffe |
Gaels |
|
| O'Keevan |
Gaels |
|
| O'Kelly |
IVL - Ireland |
|
| O'Kelly |
Laigin |
|
| O'Kennedy |
Erainn |
|
| O'Kennedy |
IVL - Galway |
|
|