Michael McKevitt, the leader of the Real IRA, was sentenced on Thursday after being found guilty by Dublin's Special Criminal Court just 24 hours earlier.
McKevitt, 53, was also found guilty of membership of an illegal organisation, although he denied both charges. He was refused leave to challenge the outcome of the case although his family said he would appeal to a different court.
The Real IRA split from the mainstream Irish Republican Army (IRA) over the 1998 Northern Ireland peace process with the British government.
It was responsible for the worst single atrocity in three decades of sectarian strife in Northern Ireland - the 1998 Omagh bombing that killed 29 people and injured hundreds of others.
One of the most spectacular hits by the group was in west London, on January 2001 when a massive car bomb exploded outside the BBC.
The Real IRA’s goals are to end British rule in Northern Ireland and return the British province to the Irish Republic.
Northern Ireland's Roman Catholic republicans want the province reunited with the Irish Republic to the south, while Protestant loyalists want to maintain links with Britain.
Michael Gallagher, whose 21-year-old son Aiden was killed in the Omagh blast, welcomed the verdict.
He said: "I'm absolutely delighted the verdict has gone the way it has… He (McKevitt) cannot orchestrate another atrocity such as Omagh."
The six-week trial depended heavily on the evidence of key witness David Rupert, a former agent of the US Federal Bureau of Investigation (FBI).
"I'm absolutely delighted the verdict has gone the way it has… He (McKevitt) cannot orchestrate another atrocity such as Omagh"
Rupert also worked for the British security services and is said to have infiltrated the Real IRA in the United States and then in Ireland.
The special court, chaired by three judges and with no jury, heard how Rupert wormed his way into republican circles during the 1990s.
However, security forces still view The Real IRA as a big threat to peace in Northern Ireland.
The group was formed by Irish Republican Army hardliners who broke away in 1997 in disgust at the involvement of the IRA's political ally Sinn Fein in the province's peace process.
It accused Sinn Fein leader Jerry Adams of betraying the Republican cause.
Security sources say the group is small, with an active membership in the dozens rather than hundreds.
Most Real IRA attacks have been carried out using homemade explosives, using technology developed over 30 years by the mainstream IRA to manufacture powerful fertiliser car bombs.
The group is estimated to possess a few dozen rifles, machineguns and pistols, a small quantity of Semtex commercial high explosive, and a number of detonators.
"The Real IRA...have probably grown stronger because there has been a slight seepage away to them (from the mainstream IRA)," said James Dingley, lecturer in terrorism and political violence at the University of Ulster.
"They never went away, they were quiet for about a year because they were getting their internal security sorted out."