Co Louth man Michael Campbell has been found guilty of trying to buy weapons and explosives for the Real IRA in Lithuania.
He was arrested in January 2008 following a sting operation involving British, Irish and Lithuanian intelligence agencies.
Judge Arunas Kisielus sentenced Campbell (39) to 12 years in prison for weapons offences and supporting a terrorist group.
Campbell, from the Upper Faughart area near Dundalk, Co Louth, denied the charges and claimed he was the victim of entrapment.
Video footage and intercepted communications showed that Campbell paid deposits of €6,000 for high-grade explosives, grenade launchers, detonators, AK-47s and a special assassin’s rifle to Lithuanian agents posing as arms dealers during a sting orchestrated by MI5.
The case centred on an undercover agent, known as Robert Janine, who for years provided his MI5 handlers with vital intelligence about the activities of dissident republicans’ plans.
In an audio recording played to the trial, Campbell is heard discussing how easy it would be with the type of equipment on offer to plant a bomb in London and escape. "You can imagine us getting over to England ... you imagine, with a six-hour timer, we could be over to London and back," he says in an audio clip after mulling over a price list for explosives and detonators. "Just tick, tick, tick, tick ... gone."
At the final defence hearing of his two-year trial last month, Campbell reiterated that he had no connection with the Real IRA – in which his brother Liam is suspected of being a senior member – and that he had been entrapped by British, Irish and Lithuanian intelligence agencies.
"I am not guilty," Campbell said of charges of attempted arms smuggling, supporting a terrorist group and illegal possession of weapons."During my entire stay in Lithuania, when I was recorded 24 hours a day, the prosecutor did not even get one recording where I was talking with anybody about terrorist acts."
Speaking after the verdict, Campbell's lawyer Ingrida Botyrienė said she would appeal the verdict.
Lithuania is also seeking the extradition of Liam Campbell and Brendan McGuigan of Omeath, but Michael Campbell insisted that they were not involved in his visits to Lithuania.
Liam Campbell is seeking to go before the Supreme Court in London to challenge a civil court ruling which held him liable for the 1998 Omagh bomb, which killed 29 people including a woman pregnant with twins.
Liam Campbell, a farmer from Upper Faughart, Co Louth, has been in custody in Northern Ireland since May 2009 when he was detained on foot after crossing the border. His case is due to by mentioned in the Laganside Courts Complex in Belfast on November 18th.
An extradition hearing against McGuigan, of Omeath, Co Louth, is expected to be heard in the High Court in Dublin next week.
Irmantas Mikelionis, chief prosecutor of the Organised Crime and Corruption Investigations Department of the Prosecutor General's Office, said the explosives could have been used for a number of attacks.
"If we failed to stop Mr Campbell, we would put in danger the lives of innocent people," he said.
Mr Mikelionis said undercover officers had risked their own lives during the sting. "The dangers of being disclosed, the danger of being accused for co-operation with secret services were hanging in the air each time they contacted the members of the terrorist group," he said. "We are happy we had no painful consequences."
Secretary of State for Northern Ireland Owen Paterson said Campbell's conviction was another excellent example of what can come from the high levels of co-operation between countries across Europe and beyond in combating
terrorism.
"I am very pleased with this verdict and would like to congratulate the Security Service and the Lithuanian authorities who have collected the evidence that the court in Lithuania has clearly found to be both convincing and compelling," he said. "I have no doubt that this will have dealt a blow to RIRA but we are not complacent and we remain determined to do all that we can to bear down on those who are the enemies of the whole community throughout the United Kingdom."