Michael Campbell was arrested in an undercover operation in the Baltic state after handing over 10,000 euro (£8,600) to buy a sniper rifle, detonators and timers,
He was remanded in custody for a further three months because of the seriousness of the charges and amid fears he would flee the country.
The trial begins in Vilnius on October 7.
Lithuanian prosecutors alleged Campbell attempted to negotiate a deal for guns, ammunition and bomb-making materials for the dissident republican group.
He faces up to 20 years in prison if found guilty.
Irmantas Mikelionis, deputy chief prosecutor of Vilnius, confirmed Campbell would be held in jail for at least another three months as he awaited trial.
The defendant is the brother of Liam Campbell, who was successfully sued by the relatives of the Omagh victims for the 1998 bomb attack which killed 29 people.
The pair are from the Upper Faughart area near Dundalk, Co Louth, in the Irish Republic, close to the border with Northern Ireland.
Michael Campbell, 36, is charged with supporting a terrorist group by attempting to smuggle firearms, ammunition, and explosive devices from Lithuania to Ireland.
He was arrested in January 2008 following the operation involving the British, Irish and Lithuanian intelligence agencies.
It is alleged he paid undercover Lithuanian agents 10,000 euro for the weapons including a sniper rifle, detonators, timers and about 10kg (22lb) of high explosives, in a garage in the industrial zone of Vilnius.
It is also claimed he illegally carried and fired from a rifle, an AK-74 and pistols and examined a sniper rifle, mechanic timers and high explosive trotyl.
Prosecutors will outline the case against Campbell on October 7 and he will then be called to give evidence.
British and Lithuanian secret agents will give evidence behind closed doors later in the month.
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