The protected witness Charles Bowden has been released from prison and is starting a new life abroad, it is understood.
Bowden (38) was the first person admitted to the State's witness protection programme, established in the wake of the murder of journalist Veronica Guerin in 1996.
He gave evidence against drug dealer John Gilligan and three other members of Gilligan's gang: Paul Ward and Brian Meehan, both now serving life sentences for Ms Guerin's murder, and Patrick Eugene Holland.
Bowden was serving a six-year sentence for drugs and firearms offences, having been granted immunity from prosecution for the murder. He is believed to have been released from Arbour Hill prison in Dublin on Tuesday. Two other members of the witness programme, Russell Warren and John Dunne, were released earlier this month.
Bowden confessed in evidence to cleaning and loading the gun used in Ms Guerin's murder, claiming he had no knowledge of the plan to kill the journalist.
But the judge in Paul Ward's trial described Bowden as "a vicious criminal. . .who would lie without hesitation and regardless of the consequences for others if he perceived it to be in his own interest to do so".
The case also heard evidence that Bowden hosted a "raucous" party at his north Dublin home on the evening following Ms Guerin's murder. And in Meehan's trial, the defence argued there was more hard evidence linking Bowden with the murder than any other member of the suspected team.
Gilligan was cleared last month of involvement in Ms Guerin's death, but the evidence of Bowden and others helped convict him of drugs charges for which he is serving a 28-year jail sentence.
Born in Finglas, Dublin, Bowden served in the Army for six years in the 1980s before being discharged over an alleged assault.