In early January 2000, a man out walking his dog along the Grand Canal at Karneystown Bridge in Co Kildare came across a man’s naked body floating in the water. A day later, as Garda divers searched the canal for a murder weapon, they pulled a second man’s body from the water.

Darren Carey (20) and Patrick Murray (19), low-level criminals who had got involved with a drugs gang based in Ballyfermot, Dublin, had both been shot in the head at close range. One of the men was said to have been sexually assaulted before being killed.

The chief suspect for the murders was Mark Desmond, a then 27-year-old enforcer who was involved in a drug-dealing operation in Ballyfermot and had already acquired a reputation for brutal violence. He was shot dead in Lucan on Friday night with sources saying there was a host of possible suspects, as many people would have wanted Desmond dead.

Back in 2000, the Garda’s hypothesis was that Carey and Murray, having been caught with Desmond’s drugs on a flight from Amsterdam to Dublin, were deemed such a liability by the young drug-dealer that he decided to kill them both.

After offering witness protection to a number of his other contacts, including an ex-girlfriend, gardaí gathered enough evidence to charge Desmond in relation to the double-killing. His trial was due to begin 2002, but as it was about to begin, a number of witnesses refused to give evidence. The State dropped the murder charges and, after a trial in which Desmond represented himself, he was convicted for firearms offences.

When the guilty verdict was read out, relatives clapped and cheered while others shouted: “You murderer.” Another shouted: “I hope Darren comes back and haunts you.”

As the judge sentenced Desmond to eight years in prison, relatives screamed: “You murdering pig, murdering pig”. Another roared: “You’re only brave with your guns in your hand. Two children dead over your drug involvement.” Desmond, smiling, replied: “Eight years. I’ll do it on my back.”

Desmond would serve much less than that. In December 2004, the Court of Criminal Appeal overturned his conviction and Desmond walked free.

In the years after his release, Desmond became a more important player in the Dublin drugs world, aligning himself with a gang that was responsible for selling large quantities of heroin in the capital. His role as enforcer and suspected hitman put him in the frame in a number of murder inquiries in the past decade.

In 2002, he was suspected of involvement in the killing of Maurice “Bo Bo” Ward (56), a former armed robber and hijacker who was believed to have been murdered in revenge for another killing. Ward was shot in the chest with a shotgun when he opened the door of his home in Ronanstown, west Dublin, and then shot in the back as he lay on the ground.

More recently, Desmond’s gang was suspected of the killing of James Kenny McDonagh (28), who disappeared in October 2010 and was filed as a missing person until his skeletal remains were discovered in the Dublin mountains six months later. Gardaí believe a number of Kenny McDonagh’s associates were members of Desmond’s gang and suspect he must have crossed the group before his killing.

As Desmond’s notoriety grew, so did his list of enemies within the networks of rival gangs vying for supremacy in the Dublin drugs underworld. He survived an attempt on his life in 2005, when a hitman on a motorcycle fired shots into his car as he waited at traffic lights in Tallaght.