Gardaí are investigating an assault that occurred at a property in Ballyhaunis on Saturday night.
The incident took place in the Hazel Lawns area of the town between 10 o'clock on Saturday night and 4 o'clock Sunday morning.
A man in his 40s suffered serious head injuries during the incident and was taken to Mayo University Hospital.
He has since been transferred to Beaumount Hospital where his condition is described as critical.
A man in his 20s was arrested at the scene and is currently detained at Claremorris Garda Station.
Anyone with any information in relation to the incident is urged to contact Claremorris Garda Station at 094 9372080, the Garda Confidential Line at 1800 666 111, or any Garda Station.