Gardai believe the man encountered a group of two men and three women when an altercation broke out.
A member of the group is believed to have slashed the victim in the face with a bottle.
"The victim's injuries are extensive," a source said.
The attack took place at the junction of Lavitt's Quay and St Patrick's Bridge in Cork city between 12.30am and 1.45am.
The victim was taken to Cork University Hospital, but his injuries are not believed to be life-threatening.
It's believed that the group fled the scene in the direction of Bridge Street. Officers were examining CCTV footage this weekend in a bid to establish what happened.
"Gardai would like to speak with anyone who was in the area of Lavitt's Quay/St Patrick's Bridge or anyone who may have witnessed the incident," a garda spokesman said.
Anyone with any information is asked to contact Anglesea Street Garda Station on 021-4522000.