The scene, outside a well-known hotel on the Muckross Road, has been cordoned off. The young man is believed to be from north Cork.
Gardaí are "investigating all the circumstances surrounding the unexplained death", a statement said.
Officers arrived at the scene at approximately 11.40pm on Saturday and found an unconscious male lying outside the entrance. The man was pronounced dead a short time later.
The body remains at the scene. A postmortem will be conducted at a later date and this will determine the course of the investigation, gardai said.
The scene is currently preserved for technical examination. Investigations are ongoing.
Gardaí are appealing for anyone with information to come forward. Any road users or pedestrians who were travelling in the Muckross Road area between 11pm and 11.45pm and who may have camera footage (including dash-cam) are asked to make this footage available to gardaí.
Anyone with information should contact Killarney Garda station (064 6671160), the Garda Confidential Line (1800 666 111) or any Garda station.
Separately, videos are circulating widely on social media of crowded streets in Killarney at the weekend, with some party-goers seen jumping onto a telephone box which contain defibrillators.
People are seen in the videos drinking and cheering at passing cars from crowded footpaths on the town's main street.
This is the latest in a series of crowded street scenes of partying in the tourist town being posted on social media in recent weeks.
