- O’Reilly’s Island
- Oatquarter
- Ogham
- Ogham
- Oghambaun
- Oghil
- Oghil
- Oghill Cross Roads
- Oghly Island
- Oileán Dá Bhranóg
- Oileán na Tuí
- Old Town
- Old Town
- Oldcastle
- Oldrock
- Oldstreet
- Omey Island
- Onaght
- Ooghwee
- Oran
- Orcusconny
- Ormond View
- Oughtagh
- Oughterard
- Oultort
- Ourtnagapple
- Owenbeg
- Owenboy
- Owenbristy
- Owenduff