Michael McKevitt has lost his Supreme Court appeal against his conviction for directing terrorism.
The 54-year-old, from Beech Park in Blackrock Co Louth, claimed he did not get a fair trial because his legal team had not been supplied with all the information relating to a key witness, FBI agent David Rupert.
However five Supreme Court judges ruled this morning that the conviction was safe and dismissed the appeal.
Michael McKevitt was jailed for 20 years by the Special Criminal Court in August 2003 for organising terrorist activities for the Real IRA. He was the first person ever to be prosecuted for this offence.
The Court of Criminal Appeal upheld his conviction and McKevitt went to the Supreme Court.
His lawyers argued the State failed to have an appropriate system in place to disclose documents relating to prosecution witness David Rupert.
In its ruling, the Supreme Court found the prosecution depended to an overwhelming extent on the evidence of Mr Rupert.
Mr Rupert was employed by both the FBI and the British security services and the court heard his life history displayed a somewhat questionable reputation.
However it was found that the prosecution's obligations of disclosure had been fulfilled because all reasonable efforts had been made in good faith to secure documentation and a high level of cooperation had been given by parties in response to the efforts.
It also found that the Special Criminal Court believed David Rupert, and that finding could not be interfered with.