The victim, named locally as Cathal Sweeney, was one of three men with known alcohol problems who were watching the Ireland and Wales rugby match at the time.
A row broke out and a drinking partner, aged in his 30s, assaulted the man, boxing him repeatedly in the head. Mr Sweeney suffered fatal head injuries.
The emergency services were contacted and arrived at the flat in the Ashdale Gardens area of Terenure around 5.45pm. The victim was taken to St James’s Hospital but died a short time later.
The State pathologist’s office was alerted and the scene was preserved pending a forensic examination.
A man in his 30s was arrested at the scene and taken to Terenure Garda Station for questioning.
It is understood that a third man who was there at the time was heavily inebriated and has not been able to provide much assistance.
A Garda source described the three men as “long-term” alcoholics, who were not involved in any serious crime.
He said the victim was known to gardaí, who had dealings with the three men in terms of drunkenness and public order.
“You are dealing here with serious alcoholism, fights, some mental health issues,” a Garda source said. “The man received a number of blows to the head with fists. He received serious injuries to his head as a result.”
Anyone with information is asked to contact 01 6666400, the Garda Confidential Line 1800 666 111 or any Garda station.