It happened at a B&B on the Kennelsfort Road Lower in Palmerstown at a quarter to four this morning.
Two men in their mid twenties were taken to hospital in Blanchardstown, and one is said to have suffered serious injuries.
The man being questioned was detained at the scene and is understood to be in his mid thirties – he's being questioned at Ballyfermot Garda station.
The scene of the incident is still sealed off and traffic diversions remain in place from Kennelsfort Road Lower into Palmerstown Village.
