O
- (County Galway) Oldstreet
- (Sligo) Owenbeg
- (Sligo) Owenykeevan
- (Sligo) Oyster Island
- (County Leitrim) Oghill Cross Roads
- (County Galway) Oatquarter
- (County Galway) Owenbristy
- (County Galway) Ower
- (County Galway) Oultort
- (Sligo) Old Town
- (County Galway) Oxgrove Bridge
- (County Galway) Oghil
- (Roscommon) Old Town
- (County Galway) Oghly Island
- (Mayo County) Oughtagh
- (Mayo County) Owenboy
- (Mayo County) Owenduff
- (Mayo County) Ooghwee
- (County Galway) Oghil
- (Mayo County) Oxford
- (Sligo) Ogham
- (Sligo) Oghambaun
- (Sligo) Oldrock
- (Sligo) Ogham
- (County Leitrim) Orcusconny
- (County Galway) Oileán Dá Bhranóg
- (County Galway) Oileán na Tuí
- (Mayo County) Owenduff
- (County Galway) Ourtnagapple
- (County Galway) Ormond View