Louth man Michael Campbell has been sentenced to 12 years in prison by a Lithuanian court for attempting to smuggle arms from the Baltic state to paramilitary group the Real IRA.
"The court has decided to impose a sentence of 12 years," trial judge Arunas Kisielius said as he read out the verdict.
Campbell, who had repeatedly insisted he was framed by British and Lithuanian intelligence, was convicted on three counts: attempted smuggling, attempting to aid a terrorist organisation and illegal possession of weapons.
He was arrested in Vilnius in January 2008 after an international surveillance operation involving MI5 and Lithuanian intelligence.
Evidence was given that he had been overheard talking about planting bombs in London.
Lithuanian prosecutors had asked the court to sentence Campbell to 16 years in jail, including his pre-trial detention, on charges of arms smuggling and aiding a "terrorist organisation".
The prosecutor said Campbell paid undercover agents €10,000 to buy weapons, including a sniper rifle, detonators, timers and high explosives, which could have been used against government targets in Britain.
Campbell told the court he had travelled to Lithuania to buy weapons, but said they were meant to be sold to criminals, not the Real IRA.
Secretary of State for Northern Ireland Owen Paterson has welcomed the court's decision.
He said: "This is another excellent example of what can come from the high levels of co-operation between countries across Europe and beyond in combating terrorism.''
Mr Paterson said he had no doubt the verdict will have dealt a blow to the Real IRA.