Colm Murphy has been found guilty of conspiring to cause the Omagh bomb that killed 29 people and two unborn children in 1998. He is the first person to be tried in connection with the bombing.
The verdict was delivered this afternoon after a 25-day trial at the Special Criminal Court, presided over by Mr Justice Robert Barr.
Delivering the verdict, Mr Justice Barr described the Omagh bombing as the "worst atrocity in Northern Ireland since 1969". He also said Murphy's admission of guilt in the course of Garda interrogation had played a part in the Special Criminal Court's finding.
He said Murphy was a "republican terrorist of long standing" and a person likely to be involved in terrorist activities of the sort charged against him.
Murphy's defence counsel, Mr Michael O'Higgins, asked for an adjournment on sentencing until Friday at 11 a.m. "There are matters within the judgment that cause us grave, grave concern," he said.
Mr Justice Barr granted the adjournment but said the court never reviewed judgments once delivered. "That is the end of it as far as the court is concerned," he said.
Murphy, from Dundalk, Co Louth, had denied one charge of conspiring to cause an explosion likely to endanger life or cause serious injury to property between August 13th and 16th, 1998.
Twenty-nine people, including a woman heavily pregnant with twins, died and hundreds more were injured when the blast tore through the Co Tyrone market town on August 15th, 1998.
The court heard that Murphy lent two mobile phones to an associate in advance of the bombing. Witnesses told the court that Murphy's phone, and that of his foreman and second cousin, Mr Terence Morgan, were tracked travelling from Co Louth to Omagh and back again on the day of the bombing.
Mr Morgan initially claimed Murphy had asked to borrow his mobile phone because his own was "on the blink". But on the penultimate day of the trial, Mr Morgan withdrew his evidence implicating Murphy, accusing gardaí of pressurising him into making the allegations.
Instead he said he had lost the phone over the weekend and found it in his van the following Monday.
During the trial the court ruled that two detectives had fabricated evidence against Murphy by adding details to a statement and making alterations.
They were described as "discredited witnesses" in a court ruling, but a defence application to have the case thrown out was rejected, with the judges deciding the actions of the two officers had not tainted the entire case.
Additional reporting by PA