Are you talking about buying a property to rent out in full or a PPR to rent rooms under the owner occupier rent a room scheme with you as a live in landlord?
You mention rooms to rent, would you be a live in landlord/renting each room individually/renting house to a person/family/couple?
Douglas is an extremely popular place to live in Cork. Obviously though the quality of the place you are going to buy, it's exact location (Donnybrook/Grange != Douglas) and the price you would look for will all be factors.
Generally places go pretty quick there, if it's a decent place it should rent quickly.
Possibly the strongest rents in Cork suburbs as Douglas has excellent amenities. A search on daft should give you a good idea of what rent you could expect.