26-year-old Dariusz Wiskowski was brought to the Mid Western Regional Hospital shortly after midnight but died at 8am on Monday.
Gardaí are satisfied that the Polish national — who worked as a fork lift driver in Annacotty — was assaulted while attending a house party at Briarfield, Castletroy.
Friends carried the unconscious victim to a neighbouring house not realising the extent of his injuries but later called an ambulance when they became concerned for his well being.
The State pathologist, Dr Marie Cassidy, has carried out a post mortem.
The man heading the investigation, Superintendent John Kerin, said there was a suggestion that the dead man had fallen down stairs during the party.
Supt Kerin said: “Because he was assaulted does not mean he was murdered. Until we get the whole picture from the final post mortem results, which may take some weeks, we are treating this as a suspicious death.
“However, until we have all the post mortem results we are not saying it is a murder investigation.”
Eight men and one women from Poland and Lithuania attended the party at which Mr Wiskowski received his fatal injuries.
Gardaí suspect an incident in which Mr Wikowski was assaulted may have occurred shortly before midnight and are satisfied that no weapon was used in the assault.
Mr Wiskowski, who was single, recently moved from Briarfield to a house in Milford Grange, Castletroy. His family in Poland have been notified of his death.
Supt Kerin revealed that all those who attended the party had been interviewed and that some of those questioned had been quizzed again yesterday. Nobody has been arrested.
Zdenek Malich, a Czech national living in the estate, said: “I was working until three. When I came home there were police all over the place. It’s usually quiet here.”