The incident took place at the entrance to a car park of a licensed premises on Clonsilla Road.
A man in his 20s has been arrested in connection with an assault in Clonsilla, Dublin last night.
Shortly after 11 o'clock, Gardaí and emergency services responded to reports of an incident at the entrance to the car park of a licensed premises on Clonsilla Road.
A man in his 40s sustained serious injuries and was taken to Connolly Hospital Blanchardstown, where he remains in critical condition.
Gardaí are appealing to anyone who was in the area at the time of the incident to come forward.