The Revolutionary Armed Forces of Colombia, or Farc, has been waging a war against the Colombian government since 1964.
It started as an offshoot of the Colombian Communist Party trying to protect the rights of peasant farmers in the countryside far from Bogotá. The conflict here is the longest-running in Latin America and the Farc is the oldest insurgency.
Despite recent blows by the armed forces since President Álvaro Uribe came to power in 2002, it is still the biggest guerrilla movement in the region.
Mr Uribe, a close Washington ally, was elected on promises of crushing Colombia's insurgency and strengthening democratic security in this war-weary Andean nation.
Right-wing paramilitaries are also involved in the conflict which claims more than 3,000 lives each year, mostly civilians.
In recent months, the FARC has stepped up its campaign of attacks aimed at undermining President Uribe's re-election attempts next year.
President Uribe has been pursuing an unprecedented, US-backed build-up against Farc rebels. Military commanders here claim the Farc has been forced into decline.
The Farc gets its funding from extortion, kidnapping and drug trafficking to the tune of billions of dollars every year.
The three Irishmen were arrested in August 2001 on their return from a Farc-controlled safe haven in Colombia's southern jungles.
In the months before and immediately after their arrest, it was widely claimed here that the Farc's military strength and bomb-making capabilities had greatly improved.
Several bombs in towns and villages around the country showed signs that the Farc had changed its methods and authorities said this was due primarily to training given by the Irishmen.
Military commanders claimed the rebel group had received training in urban warfare tactics and instruction in bomb-making skills from the IRA.
On August 7th, 2002 mortars were fired at the presidential palace in downtown Bogotá during Álvaro Uribe's swearing-in ceremony.
They fell short of their target and killed more than 20 people in a nearby poor neighbourhood. Authorities claimed that the tubes the mortars were fired from bore a strong resemblance to those used by the IRA in Northern Ireland.
At the trial of the three Irishmen, these claims were refuted by a leading British forensic expert, Dr Keith Borer.
Despite recent government claims that the Farc is on the decline, attacks over the past few months have damaged the Colombian army's offensive in several rural areas.
The southern state of Putumayo, a traditional Farc stronghold is currently at a standstill, and it is believed that the rebels may be trying to attract the limited army resources away from other areas of the country in order to mount more attacks.