Four men were charged with murdering Carroll, but only one – gunman Peter Kenny (29) from Rialto, south Dublin – was convicted of the horrific crime after the evidence of State supergrass Joey O'Brien, who is now in the witness protection programme.
The Herald can reveal that Carroll was murdered after one of the country's top gangland criminals accused him of having an affair with his ex-girlfriend.
"This guy is a psychotic, evil b**tard who had Carroll murdered because he wrongly thought Carroll went off with that girl," said a senior source.
It has emerged that in the months before Carroll was shot dead, he was subjected to death threats from the gangster.
Carroll, who was linked to a number of the city's major drug dealers, was so paranoid about the threats that he stayed in hotels in the weeks before he was shot dead in the south inner city pub.
Gardai believe the suspected drugs trafficker was set up by some of his closest associates in the murder that led to two lengthy and hugely controversial trials.
In the first trial, which concluded in July 2011, Peter Kenny (31), of McCarthy's Terrace, Rialto, smirked as he was found guilty of the murder after six weeks of evidence.
Bernard Hempenstall (28), from Park Terrace, The Coombe, and Damien Johnston (29), of Cashel Avenue, Crumlin, were acquitted, and no verdict was reached in relation to Christopher Zambra (38), of Galtymore Road, Drimnagh. The DPP then decided to retry Zambra, who was cleared in June this year.
The case centred around the evidence of drug dealer Joey O'Brien. O'Brien testified that he had two meetings with Christopher Zambra and others in the days before the murder.
He said John Carroll's murder was planned during these meetings and that Zambra was offering €30,000 for his killing.
O'Brien said that Zambra asked him to source a motorbike to use in the killing and to burn it out afterwards. The jury saw CCTV footage of the arrival of the motorbike at the pub around 9.30pm that night and of the shooting itself.
The motorbike pulled up outside and the passenger, wearing a helmet, went inside and immediately identified Mr Carroll. He shot at him with a pistol and Mr Carroll darted across the pub in an attempt to escape.
However, he was shot a number of times before being taken to hospital, where he died.
O'Brien had admitted his involvement in the killing, but was never charged.
Convicted armed robber Zambra hugged his family and members of his legal team before walking free following the two-month trial at the Central Criminal Court.