Thomas Cunningham, 46, was found in the garden of his parents’ house on the outskirts of Ballinlough on Thursday morning.
A postmortem examination later that night showed the father-of-one died as a result of an assault.
A murder investigation was launched by gardaí in Castlerea and the youth was arrested early yesterday morning.
Mr Cunningham had returned to Ireland from England with his teenage son six years ago to look after his elderly parents.
He lived with them in a bungalow on the outskirts of the quiet village of Ballinlough, located where the counties of Roscommon, Mayo and Galway meet.
Gardaí received a phone call early on Thursday morning and arrived at the scene at around 9am. They found the body of a man slumped in the front garden of a house.
The scene was immediately sealed off and an investigation commenced.
The area was examined by officers from the Garda Technical Bureau, while State Pathologist Dr Marie Cassidy carried out a preliminary post mortem at the scene.
She carried out a full post mortem later at Portiuncula Hospital in Ballinasloe.
“As a result of a post mortem carried out by the State Pathologist Dr Marie Cassidy we have launched a full murder inquiry at Castlerea Garda Station,” said Superintendent Sean Ward.
“I can confirm the deceased man died as a result of assault.”
He declined to comment on the nature of the injuries for operational reasons as doing so might affect the investigation.
Gardaí said they could not yet confirm when exactly Mr Cunningham died.
The death has shocked locals in the area.
One neighbour described him as a “quiet man”, who didn’t speak very much.
Another neighbour said: “It’s very sad for the family, they have a very wide network of family, which will have a knock-on effect of tragedy.”
Anyone who was in the Ballinlough area between 5pm on Wednesday and 9am on Thursday, and who may feel they saw something suspicious or relevant to this investigation, is asked to contact Castlerea Garda Station on 094-9620019.
Alternatively, they can contact the Garda confidential line on 1800 666 111 or any Garda station.