GARDA� investigating the gangland execution of a Dublin drug dealer believe that close associates of a man shot dead outside Cloverhill Prison in 2004 paid a hitman to murder him.

Detectives are certain that a professional killer was behind the murder of 25-year-old Robbie O'Hanlon, who was shot three times in the head in front of more than a dozen shocked witnesses after a soccer match in Clondalkin on Wednesday.

Garda� are working on the theory that friends of Jonathan O'Reilly, who was shot dead outside Cloverhill in April 2004, organised the hit because O'Hanlon set up his murder.

O'Hanlon drove O'Reilly to the spot where he was murdered and left the car, claiming he was going into the prison to see somebody. He was only gone a matter of minutes when a motorbike drove up to the car and two men shot O'Reilly dead. It was widely believed that O'Hanlon was responsible for organising the murder, even though he always denied it.

It is thought that O'Reilly's friends waited nearly three years to get their revenge because they wanted to throw garda� off the scent. O'Reilly (25) had a number of relatives who are involved in serious crime and some of his friends are also well-known criminals.

Robbie O'Hanlon was a small-time dealer described as a "thug" with a lot of enemies, although it is not suspected that his involvement in the drugs trade was the reason for his death. He was involved in a drug gang that distributes cocaine and heroin around the Tallaght and Clondalkin areas.

O'Hanlon was well known to garda� and had a number of previous convictions, mainly for assault. He was originally from Arthur Griffith Park in Lucan but lived in an apartment in Parkwest Pointe in Clondalkin.

Senior garda sources are convinced that it is the O'Reilly connection that led to the third gangland murder of 2007.

A detective said: "Some people are saying that if one of Jonathan O'Reilly's associates wanted O'Hanlon dead, they would have done it before now. That's not necessarily true. There is an old saying that revenge is a dish best served cold and they could have waited until now to put other people in the frame.

"O'Hanlon and O'Reilly were good friends before Robbie set him up and took him to die. People don't forget that.

There is no doubt that whoever murdered O'Hanlon was a professional who had killed before. He stalked his target and waited behind a tree for him to finish his football match.

"It takes a professional to calmly walk up to somebody and blow their brains out, especially when there are lots of witnesses standing only yards away. Whoever was responsible knew exactly what they were doing and was probably well paid for it."

Two men, Thomas Hinchon (25) from Clondalkin and Finglas man Brian Kenny (36), were both convicted of Jonathan O'Reilly's murder. Robbie O'Hanlon was a key witness during the trial in 2005. He was asked by defence barrister John Phelan if he deliberately parked his car facing the wrong direction in order to leave O'Reilly exposed so he could be easily murdered.

An angry O'Hanlon denied this and said: "Watch your mouth. This fella is making accusations against me. He's also putting my life in danger. You're putting ideas in people's heads. You're brain dead. You're talking s**t."

Another barrister asked him how he could afford two cars worth nearly Euro20,000 when he did not work. O'Hanlon told him to mind his own business and said: "Don't f**king raise your voice at me, you f**king thick."

O'Hanlon was released from prison last summer following a two-year sentence for assaulting his former partner, the mother of his twin girls. He broke into her home and beat her with a can-opener and sweeping brush until the handle broke. He then held a knife to her throat.

Garda� do not believe that his murder will lead to retaliations. "O'Hanlon was a thug who did not have many friends.

He will not be missed by many people except his unfortunate family. That is the reality of the situation, " said one.