A police spokesperson said an ambulance was called to the incident in the Cashel St/High St area at 3.33am.
"The man was found injured, and was transported to hospital with serious head injuries," said the spokesperson.
"Police are working to establish the circumstances of the incident and have spoken to a number of people, however, we know there are others who are yet to speak to us."
In particular, police are seeking the man shown in CCTV footage above and the individuals who assisted ambulance staff on the night.
"We believe these people may have valuable information that would assist with our enquiries," said the spokesperson.
Anyone who witnessed this incident or knows anything that could help with the police investigation is urged to phone 105 and quote file number 200309/5599.