Convicted drug trafficker John Gilligan's prison sentence expires at midnight tonight.
The 61-year-old was jailed in 2001 for 28 years for possession of commercial quantities of cannabis resin. The sentence was reduced by 8 years in 2003 on appeal.
In 2001 he was acquitted of the murder of journalist Veronica Guerin.
Gilligan has been in custody since 1996.
His prison term expires at midnight, but it is unclear if he will be released immediately or at some stage tomorrow morning.
Gilligan has also lost a Supreme Court challenge against his sentence on the eve of his release.
He took issue with the mandatory nature of the consecutive sentences he received for offences he committed as an inmate.
While serving his 20-year-term for drug offences, he was given a further two years for assaulting a prison officer and another six months on top of that for possessing a mobile phone.
The case won't have an impact on his release tonight or tomorrow morning.