| Families |
Ethnic Group |
Branch of |
| O'Quin |
Erainn |
|
| O'Quin |
IVL - Ulster |
|
| O'Quinlan |
Gaels |
|
| O'Rahilly |
Gaels |
O'Neill (Ulster) |
| O'Regan |
Gaels |
|
| O'Regan |
OVL - Limerick, Cork |
|
| O'Reilly |
Gaels |
|
| O'Riordan |
|
O'Carroll |
| O'Rourke |
Gaels |
|
| O'Scanlan |
Gaels |
|
| O'Scanlan |
IVL - Sligo |
|
| O'Shanahan |
Erainn |
|
| O'Shaughnessy |
Gaels |
|
| O'Shea |
Erainn |
|
| O'Sheehan |
Laigin |
|
| O'Sheridan |
Gaels |
|
| O'Shiel, O'Sheill |
Gaels |
|
| O'Sullivan |
Gaels |
|
| O'Sweeney |
|
MacSweeny |
| O'Toole |
IVL - Ulster |
|
| O'Toole |
Laigin |
|
| O'Tracy |
|
MacGorman |
| O'Tracy |
IVL - Galway |
|
| Ogilvy |
Cruithne |
|
| Oliphant |
Normans |
|
| Oynie |
|
Innes |
| Parlane |
|
MacFarlane |
| Patrick |
IVL - Ulster, South Scotland |
|
| Paul |
|
MacKay |
| Paul |
|
MacPherson |
| Polson |
|
Ross |
| Power |
Normans |
|
| Prendergast, Pendergast |
Normans |
|
| Purcell |
Normans |
|
| Quillen |
|
O'Cullane |
| Quillen |
|
O'Cullen |
| Quillen |
IVL - Ireland |
|
| Quinlivan |
|
O'Quinlan |
| Ramsay |
Normans |
|
| Rattray |
Cruithne |
|
| Reardon |
|
O'Carroll |
| Redmond |
|
Burke |
| Redmond |
IVL - Ireland |
|
| Reid |
IVL - South Scotland, Ireland |
|
| Reid |
|
Ramsay |
| Reidford, Redford |
|
Innes |
| Revie |
|
MacDonald |
| Reynolds |
|
MacRannall |
| Reynolds |
IVL |
|
|