Former Real IRA leader Michael McKevitt has failed to have his conviction overturned in the Irish Republic's Court of Criminal Appeal.
Michael McKevitt sought to discredit a vital witness
McKevitt was sentenced to 20 years in 2003 for directing terrorism and membership of an illegal organisation.
His lawyers had sought to have his conviction quashed by challenging the credibility of David Rupert, the main prosecution witness.
Mr Rupert was a secret agent for the FBI and the British secret service.
McKevitt, 54, from Blackrock, County Louth, was the first person to be convicted in the Republic for the offence which was introduced after the 1998 Real IRA bomb attack in Omagh.
The explosion claimed the lives of 29 people, including a woman pregnant with twins.
He also received a six years concurrent prison sentence for membership of an illegal organisation which the court said was the Real IRA.
Mr Rupert was reported to have infiltrated the Real IRA and attended Real IRA Army Council meetings where McKevitt was present.