Michael McKevitt has indicated he will appeal after he was found guilty by the Special Criminal Court of directing the activities of an illegal organisation.
McKevitt, who is 53, and from Blackrock, Dundalk, in Co Louth, was the first person to be charged with directing terrorism, an offence created under legislation brought in after the Omagh bombing. He was also found guilty of membership of the Real IRA.
During the trial, McKevitt denied both charges.
The judges will pass sentence at 2pm tomorrow. He could face life imprisonment
McKevitt refused to leave his holding cell at the court where judgment was given following a five-week trial.
The Court said it was satisfied that the main prosecution witness, David Rupert, was a truthful witness and that he had been with McKevitt on a number of occasions.
It said McKevitt's instructions to him as regards being the organisation's US representative and acquiring equipment for him constituted 'directing' under the act.
It also said Rupert's evidence was corroborated by other witnesses and by the discovery of a computer and a guide book to Yugoslavia in McKevitt's house.
It rejected McKevitt's claim that he never met him.
FBI agent David Rupert had told the court about army council meetings he attended with him and plans that McKevitt had to launch a new wave of shootings and bombings in the North and in Britain.
However, when a Garda surveillance document was disclosed to the court, McKevitt's defence claimed it contradicted evidence Mr Rupert had given under oath, and applied to have the trial stopped.
When this was refused, McKevitt sacked his legal team and refused to participate any further. He called it a political showtrial and stopped attending court.
The remaining witnesses, including the Garda Chief Superintendent who testified that he knew the 53-year-old to be a member of an illegal organisation, gave their evidence to the court unchallenged.
Detective Chief Garda Superintendent Martin Callinan said the outcome was significant for those engaged in the job of investigating terrorism.
But he said there was still a continuing threat to the democratic process in Ireland.
Mr Callinan said that FBI agent David Rupert had been a key witness and praised his contributions to the proceedings.