The collision took place at Corn Market Row at approximately 1am on Sunday morning.
Three pedestrians, one man and two woman, were struck by a vehicle which left the scene. All three were removed from the scene by ambulance to University Hospital Limerick.
The male pedestrian (33) received serious injuries and is in critical condition. The two female pedestrians, both aged in their late 30s, received non-life threatening injuries.
A fourth female pedestrian (19) presented at University Hospital Limerick also injured by the same vehicle at the same time. Her condition is non-life threatening.
The offending vehicle was located abandoned a short distance away and has been seized for technical examination.
Gardaí are appealing to anyone who may have witnessed this collision to come forward.
Anyone with any information is asked to contact Henry Street Garda station on 061 212400, the Garda Confidential Line on 1800 666 111, or any Garda station.