Michael Campbell, 39, a member of the Real IRA, appeared cheerful as he was sentenced Friday, the British newspaper The Daily Telegraph reported. He raised his handcuffed wrists as he was led away.
Campbell was arrested in 2008 after he agreed to buy weapons and explosives from Lithuanian undercover agents posing as dealers. His trial took two years.
Campbell's brother, Liam, who was found responsible in a civil action for the deadly 1998 Omagh bombing, and Brendan McGuigan, who was arrested in Ireland, are fighting extradition to Lithuania.
The investigation was originally targeted at cigarette smuggling, which is believed to be a major source of Real IRA financing. The British domestic intelligence agency MI5 recruited an English businessman involved in smuggling who agreed to become an undercover informant.
Irmantas Mikelionis, chief prosecutor of Lithuania's Organized Crime Department, said the informant showed great courage and is likely to have saved the lives of innocent people.