Paramedics alerted Gardaí after they were called to deal with a man who had suffered head injuries at a house in Willowbrook Park at 10 to 9 last night.
He was pronounced dead shortly before 10:30 pm.
A man in his 20's is still being questioned about the incident.
The scene remains sealed off ahead of the arrival of the Deputy State Pathologist Dr Michael Curtis and the Garda Technical Bureau.
