'Champagne Killer' Karl Breen died after taking a cocktail of pills, including sleeping tablets.
Breen (34) had become increasingly paranoid in recent weeks and believed that gardai and his former associates were determined to have him murdered, sources have said.
The gang leader was growing increasingly confused and was watching over his shoulder. He referred to one incident with gardai which he wrote about online shortly before Christmas.
"They tried to kill me and make it look like an accident, only for there was a witness there they would've killed me," Breen posted on Facebook.
Sources say that these comments show just how paranoid he had become in his last few weeks alive and that the former gang boss "saw enemies everywhere".
The grim discovery of his body was made by Breen's sister Kim, her partner and Breen's girlfriend, Ashlee, when they forced entry into the apartment he was staying in at the Tolka Vale complex on the Finglas Road, north Dublin, shortly after 11pm on Monday.
The trio used a fire extinguisher to break into the apartment through a window after they were unable to get into the apartment through an exterior door. The notorious 34-year-old gangland figure was discovered slumped over on a seat and emergency services were immediately notified.
Gardai were quickly able to establish that Breen had not been the victim of foul play and the results of toxicology tests are expected to confirm that he died from a drugs overdose.
The dad-of-three was last seen alive on Sunday. His family and Finglas-based girlfriend became concerned for his welfare when they were unable to contact him on Sunday night and throughout Monday despite attempting to phone him dozens of times.
Breen, who ruled the Clondalkin-based 'Infamous D22' gang, was dubbed the Champagne Killer because he had been drinking bottles of Moet in a hotel room before the fatal stabbing of his friend Martin McLaughlin in 2006.
Since his release from prison where he served a lengthy sentence for the manslaughter of McLaughlin, Breen found himself increasingly isolated and ended up hanging around with low-level criminals from Athlone, Co Westmeath.
It has emerged that Breen's sense of "paranoia and isolation" increased in recent weeks.
This could be clearly seen in the foul-mouthed rant he posted on his Facebook site in December as a reaction to a Herald article which revealed that gardai were trying to locate him.
Referring to the bitter feud his gang had been involved in with a rival west Dublin mob, Breen claimed that the dangerous dispute was now over.
"Theres NO feud in clondalkin, its sorted but d media are doin their best to restart it d scumbags (sic)," Breen wrote on the site.
Gardai remained at his Finglas apartment last night but sources stress that the investigation into his death is not a criminal one.
The former gangster's funeral is expected to take place later this week when his body is released back to his family.
Gardai will monitor the event in an attempt to ensure it is not hijacked by Breen's criminal rivals like the funeral of his murdered right hand man Pierce Reid was in August, 2009.
On that occasion, the roof of the local church where Reid's funeral mass was due to take place was daubed with graffiti taunting the victim's gang.