The incident took place just after 5am this morning, close to the junction of Camden St and Harrington St. in Dublin.
A man in his 20s was assaulted by another man, also in his 20s, who ran from the scene.
The injured party was left with head wounds, and was first attended to by Dublin Fire Brigade personnel. He was then rushed to St James’ hospital, where his condition is described as serious.
The suspect is a young man in his 20s, with black hair, and wore a dark top and blue jeans. He ran off in the direction of Wexford St.
Gardaí in Kevin St. are reviewing CCTV footage and the area has been sealed off for a technical examination.
Witnesses can contact Gardai at Kevin St on 01 6669400, the Garda Confidential Line on 1800 666 111 or any garda station.