'The Infamous D22' gang led by Breen were one of the most violent mobs to operate in the history of Irish organised crime.
Gardai believe that Breen's mob were responsible for at least three murders which they carried out when the gang was one of the biggest heroin trafficking operations in west Dublin.
Eventually the gang decreased in influence once Breen was locked up in 2007 for the fatal stabbing of his pal Martin McLaughlin, a killing which gave him his 'Champagne Killer' nickname because he had been drinking bottles of Moet in the hotel room before the fatal stabbing.
Another major blow for Breen's mob happened in August 2009 when the gang's chief enforcer, Pierce Reid, was shot dead outside his Clondalkin home.
Since then, Breen's influence in Ireland's criminal underworld had waned to the point that he was no longer considered a major player when he was released from jail last October.
But things were very different in the years between 2004 and 2007 when 'The Infamous D22' gang burst onto the gangland scene.
The first murder they were suspected of was that of drug dealer Owen McCarthy (22) who was taken by his killers to the Wicklow Gap, shot in the head, and left at the scene in November, 2005.
McCarthy had owed the gang a major debt after losing drugs that were owned by the mob.
He had been the intended target of an attempted murder in a pub two months before his death, but the gunman shot an innocent man in the bar instead.
The criminal who botched the attempt to murder McCarthy in the pub was a dissident Republican from Derry called Shea Bradley who was shot dead himself at a laneway off Blackhorse Avenue in March 2006.
Less than 10 months later, in January 2007, Breen's mob are suspected of shooting dead printer Damien Dowdall (22) at his Clondalkin home in front of his partner.
Dowdall had no criminal convictions and was not known to gardai but is believed to have been involved in a bitter row with Breen and his cronies.
The same mob are also suspected of shooting Thomas McDonagh (28) dead in Clondalkin less than a month later.
Sources say that McDonagh was targeted after he stood up to Breen's cronies.