The case was notable for the four-day testimony of the agent, Patrick Daly, whose two decades as an informer illustrated the extent to which British Special Branch and MI5 have been able to penetrate areas of republican terrorism.
After a 14-day trial at the Old Bailey, Martin McMonagle, from Limerick and Liam Heffernan, from Belfast, both 31, were convicted of conspiracy to cause explosions, conspiracy to steal explosives, and possessing a firearm with intent to endanger life.
Jailing McMonagle - said to be a leading INLA figure - for 25 years, and Heffernan for 23 years, Mr Justice Sachs told them: 'You besmirched the fair name of Ireland, where good people are working towards peace in stark contrast to your activities. Your chilling intention was to maim, kill and destroy, a prospect which clearly gave you much pleasure.'
The judge did not think 'for one minute' that Mr Daly was an 'agent provocateur' who instigated and led the plot himself, as the defence suggested.
Heffernan and McMonagle left the dock shouting 'up the INLA, up the Irish people and up yours' to the judge. Their lawyers called only one witness - who accused Mr Daly of espousing violence - and took the rare step of declining to make closing speeches.
The convictions were being seen last night as proof of a successful partnership between police and MI5 which, in this case, prevented a major INLA campaign designed to demonstrate their effectiveness to the IRA. Their targets were said to include Unionist MPs, military bases, a ferry, an oil refinery and a gas terminal.
Mr Daly, a member of INLA's political wing who lived in Bristol, was recruited as an informer in 1974, when he was detained under the Prevention of Terrorism Act and threatened with deportation.
While reporting back to Special Branch throughout the 1970s, Mr Daly carried banners on demonstrations, supported the Troops Out movement, protested against the Prevention of Terrorism Act and established a branch of the Irish in Britain Representation Group. He was paid pounds 80 to pounds 100 a month, plus expenses. When he stopped working for Special Branch in 1989, he received pounds 2,180. 'I don't think it was big money, I was risking my life,' he said.
In the mid-1980s, Mr Daly notified Special Branch about Peter Jordan, a retired teacher from Bristol and a republican sympathiser, who was supplying the INLA with information about army officers. In 1986, Jordan was jailed for 14 years for his part in a plot to bomb a retired SAS officer in Hereford and Worcester. Mr Daly admitted sending more than 50 letters and messages to Jordan in prison, offering support and assistance in a campaign to win his freedom. Under cross-examination, he said he had pretended to be his friend as part of his cover.
In 1989, Mr Daly moved to Galway in the Irish Republic, where he set up a driving school business and was subsequently asked by the INLA leadership to reconnoitre West Country quarries - from which explosives could be stolen - and find a safe house.
By now, Mr Daly was being paid by MI5, although it was never established whether he went to Galway and became involved in the plot at M15's instigation. It is unclear whether the Irish authorities were notified; the judge rejected defence submissions for the trial to be halted because they were not informed. In the current climate, a protest by Dublin is extremely unlikely.
Although MI5 controllers conceded there was 'a struggle' between them and Special Branch over taking the lead role against Irish terrorism - which they were eventually given - they rejected defence suggestions that the operation was deliberately mounted to earn them credibility.
Acting under MI5 supervision, Mr Daly carried out reconnaissance for the INLA. His controllers helped him pick a quarry in the Mendips and purchased a safe house in a Bristol suburb. They wired it for sound and awaited the arrival of the three-man INLA team, whose movements were monitored from the moment they arrived in Britain. The jury heard tapes of the men discussing targets and talking about the INLA bombing of Airey Neave, the Conservative MP; McMonagle described it as 'a great job' and said they needed another like it.
At the quarry, police were waiting, but Operation Breaksea went wrong when McMonagle stepped on a marksmen in undergrowth. He and Heffernan were arrested, but the third man, Anthony Gorman, escaped. He is wanted for the murder last year in Derby of Michael Newman, an Army recruiting sergeant.
To prevent the INLA exacting revenge on Mr Daly - described by his MI5 controller, Mr C, as 'an outstanding agent' - extraordinary measures have been taken. He and his family are in hiding under new identities, created by MI5, and cushioned by a payment of pounds 400,000. With his evidence behind him, steps may be taken to change his appearance.
During Mr Daly's evidence, unprecedented measures prevented him being seen by the media, under an order not to publish any photograph or drawing. Although Mr Daly was visible to the defendants, lawyers, jury and court officials, screens and brown paper across the back of the dock kept him out of sight of the media. In the public gallery, directly above the witness box, only the rear row was used. When Mr Daly entered or left, the court was cleared.
Nigel Sweeney, for the Crown, said: 'He knows . . . his cover has been blown, he has in effect put himself in prison for the rest of his life. If he had been discovered, there would not have been for him a three-week trial at the Old Bailey, just a quick bullet in the back of the head.'