It is understood the victim, aged 25 and from the Johnstown area of Tipperary, was standing on Liberty Square in Thurles at 1.30am when he was struck in the head by another man.
He fell to the ground and hit his head on the pavement.
The man was rushed to South Tipperary General Hospital in Clonmel, where he remained in a critical condition last night.
A 22-year-old man was last night being detained under Section 4 of the Criminal Justice Act at Thurles Garda station.
Gardaí appealed for any witnesses to the incident or to anyone that may have been in the area between 1am and 2am to contact Thurles Garda station on 0504 25100.
The assault occurred in the same location at which Kilkenny hurler PJ Delaney was attacked in 1999. Mr Delaney was left in a coma for nine days.
Meanwhile, two men in their 30s were due to appear at Letterkenny District Court at 10.30am today, charged in relation to an assault on a 41-year-old man in the town on Friday.
The incident happened at Glenard Park, Letterkenny at about 11.30pm and the man suffered serious head injuries.