| Families |
Ethnic Group |
Branch of |
| O'Kiernan |
Laigin |
|
| O'Kinneally |
Erainn |
|
| O'Lalor, Lawlor |
Cruithne |
|
| O'Leary |
Erainn |
|
| O'Lee |
Gaels |
|
| O'Lonergan |
Erainn |
|
| O'Loughlin |
|
MacLoughlin (Ulster) |
| O'Loughlin |
Erainn |
|
| O'Lynch |
Cruithne |
|
| O'Lynch |
IV - Donegal, Connacht, Munster |
|
| O'Lynn |
Laigin |
|
| O'Madden |
Laigin |
|
| O'Maher |
|
O'Carroll |
| O'Mahony |
Gaels |
|
| O'Malley |
Erainn |
|
| O'Malone |
Gaels |
|
| O'Mannin |
Cruithne |
|
| O'Meara |
Erainn |
|
| O'Meehan |
IVL - Louth, Clare, Galway |
|
| O'Meehan |
|
MacCarthy |
| O'Mellan |
|
O'Neill (Ulster) |
| O'Moledy |
|
O'Farrell |
| O'Molloy |
Gaels |
|
| O'Molloy |
Gaels |
O'Connor |
| O'Molony |
Erainn |
|
| O'Monahan |
Laigin |
|
| O'Mooney |
Laigin |
O'Connor |
| O'Morchoe |
Laigin |
|
| O'More |
Cruithne |
|
| O'Moriarty |
|
O'Donoghue |
| O'Moroney |
Erainn |
|
| O'Mulconry |
Gaels |
O'Connor |
| O'Mulderry |
|
O'Donnell |
| Muldrew, Muldragh |
|
O'Mulderry |
| O'Mullally |
Laigin |
|
| O'Mullen |
|
O'Concannon |
| O'Mullen |
IVL - Ulster |
|
| O'Mulroony |
Laigin |
|
| O'Mulryan |
Laigin |
|
| O'Mulvihil |
Gaels |
O'Connor |
| O'Murphy |
|
Kinsella |
| O'Naghten, Naughton |
Laigin |
|
| O'Naghten, Naughton |
IVL - Clare |
|
| O'Neill |
Gaels |
|
| O'Neill |
IVL - South Ireland |
|
| O'Nihill |
Erainn |
|
| O'Nolan |
|
O'More |
| O'Phelan |
Erainn |
|
| O'Quigley |
Gaels |
|
| O'Quin |
|
O'Farrell |
|
