A woman was also injured in the incident and was taken to Daisy Hill Hospital but has since been transferred to the Intensive Care Unit at the Royal.
It is understood she is currently in a critical condition.
Detective Chief Inspector Geoff Boyce said: “Police were called to the incident at a house in the Lower Dromore Road area at 7.30pm on Friday, January 18.
“A female has also sustained serious injuries during the incident and has been taken to hospital.”
DCI Boyce urged anyone with information that could assist detectives investigating the incident to contact them on 101 or to contact the anonymous charity Crimestoppers on 0800 555111.
South Down Sinn Fein Assembly member Sinead Ennis last night said that the community of Warrenpoint were “shocked and stunned” by what had unfolded.
“Our first thoughts are with the family of the victim,” she said.
“The PSNI have the area sealed off and I would appeal to anyone with information to bring it forward to assist the investigation.”