The man in his 30s, who is from the area, was assaulted at Willie Andies pub at New Square in the town centre last night.
The man was from a large family and was married with three children.
It is understood a verbal dispute broke out prior to the assault which took place at around 11pm.
Paramedics rushed to the scene and armed gardaí were also deployed to support local gardaí.
Efforts were made to save the man’s life at the scene before he was pronounced dead a short time later. His body remained at the scene this morning awaiting the State Pathologist’s arrival and a technical examination was also due to be carried out.
A man in his 20s has been arrested under section four of the Criminal Justice Act and is currently being detained at Fermoy Garda Station.
Local TD Kevin O’Keeffe, who had been out in Mitchelstown last night, said the community is stunned by the incident. Mr O’Keeffe said he knew the man who had been assaulted and died.
“He was from a large family. He was married with three children. Everyone is shocked,” he said
Another local man, who did not want to be named, said there was a sense of disbelief at what had happened.
“It is not a big town and you don’t expect this kind of thing to happen in your local area,” he said.
“It is a quiet, rural town with a population of around 3,000. You never think it would happen here.”
An incident room has been set up at Fermoy Garda station and gardaí are checking CCTV and appealing for witnesses to contact Fermoy Garda Station on 025-82100.