The Herald can reveal that O'Brien is a junior member of a notorious crime gang from Bray, Co Wicklow, who is still under death threat because of his involvement in the murder.
He remains an active target for the crime gang led by jailed killer Brian Rattigan because his victim Shay O'Byrne was Rattigan's best friend and the long-term partner of Rattigan's sister Sharon.
O'Brien has been in custody since being charged with the murder by gardai from Tallaght in March 2010. Before being locked up, the former heroin addict was linked to a number of reckless criminals from Bray including a hood who is serving a sentence for drug dealing and a career criminal who is on bail for drugs charges.
The leader of this faction is a veteran crimelord who is based in Spain's 'Costa-Del-Crime' -- the crew have links to 'Fat' Freddie Thompson's mob.
O'Brien has been involved in a number of local feuds in the Co Wicklow town which have resulted in violent incidents in which he was heavily involved, including driving a car at a rival.
He was also suspected of "peripheral" involvement in a shocking double murder in Firhouse in November 2005.
O'Brien's five-week trial heard that Mr O'Byrne had been on his way out for the evening with his partner Sharon Rattigan when he was shot in the back by a hooded gunman.
Ms Rattigan had been strapping their baby son into his car seat and Mr O'Byrne had been locking the house when the attack happened.
Speaking outside the court yesterday, Shay O'Byrne's sister Charis said a "gaping hole" had been left in their family from his loss. She thanked the gardai, the jury and the legal team for their hard work, and said that although justice had been done, it would not bring her brother back. "He was a very happy-go-lucky, loving person who didn't deserve to die the way he died," she said.
The victim's cousin, Annemarie Foley, read out a victim impact statement in court on behalf of the O'Byrne family and Sharon Rattigan.
She said Shay had been a popular, fun-loving person who would always help a friend in need, and was a "constant light" in the lives of his family, partner Sharon and his two children Ricky and Jody.
She told the court that Ricky was only 23 months when he witnessed his father's murder, and that the only small mercy for the family had been that Shay's daughter Jody, then aged six, had not been present at the scene.
The court heard that Jody has not mentioned her father's name since he was killed.