Detectives are appealing for information and witnesses following the incident in the Tynan Drive area of Monkstown on Friday evening (January 20).
A man has been arrested and is police custody.
Detective Sergeant Boyd said: “Police received a report shortly after 11.40pm that a man had been knocked unconscious in the street, and had suffered substantial injuries to his head.
"The man, aged in his 20s, was taken to hospital for treatment to his injuries and remains in a serious condition.
"A 27-year-old man has since been arrested on suspicion of attempted grievous bodily harm with intent. He remains in police custody at this time.
"We are appealing to anyone who was in the area at the time, and witnessed what happened, or who may have mobile footage or information that could help with our investigation, to call detectives on 101, quoting reference number 1945 of 20/01/23.”