The grim discovery was made by a letting agent who was forced to break down the door of the property in Santry Cross to get in. The victim's body was discovered with a kitchen knife lodged in his chest.
So far gardai have been unable to formally identify the victim and have yet to talk to the second man who was sharing the apartment with him.
The victim -- who is believed to be a Dutch national of north African origin -- was sharing the apartment with another man of similar nationality, it is believed.
It is thought the victim may have been lying in the bathroom of the Linnbhla apartments since last Friday.
The man had a clerical job in Santry and was last seen there on Friday, but he failed to turn up for work this week. His co-workers raised the alarm.
The victim is in his late 30s or early 40s and had been living in the apartment for some time with a flatmate.
Today gardai were trying to confirm the identity of the man and contact his family in the Netherlands.
The grim find was made shortly after 4pm yesterday by a letting agent, who managed the first-floor apartment. He was forced to break down the door when attempts to contact the man by phone went unanswered.
The area was sealed off for examination by gardai and the office of the State Pathologist was also notified.
Pathologist Dr Marie Cassidy arrived at the scene last night and the body was removed after an initial examination. A post-mortem was due to be carried out by Dr Cassidy in the city morgue at Marino this morning.
Pending the outcome of the post-mortem, the man's death is being described officially by the gardai as suspicious.
Also known locally as the 'Polish apartments', the Linnbhla complex comprises 216 apartments with a mix of up to 30 nationalities.
Garda Supt John Moran, from Ballymun Station, is leading the investigation.
Gardai last night appealed for information from anyone who might have seen anything suspicious in the area since last Friday to contact them at Ballymun station on 01 6664400, or on the confidential line on 1800 666111.