Robert Fryers, 44, was jailed for 25 years and Hugh Jack, 37, for 20 years after they were convicted at the Old Bailey of conspiring to cause explosions.
Their plans for a bombing campaign in Britain were foiled by the first anti-IRA operation run jointly by MI5 and the police.
The case, however, has raised serious questions about future trials involving MI5 after details of the operation had to be kept secret for "security reasons".
Sentencing the men, in what may be one of the last IRA trials, Mrs Justice Ebsworth, said: "You are both shown to be ruthless and remorseless men -prepared to wreak havoc in people's lives, as well as economic destruction."
Armed police ambushed Fryers, from Belfast, in July 1993 as he waited at a bus stop in London with a bomb in his bag. The device, which police believe was intended to be planted in the capital's financial centre - possibly a pub - was made of two-and-a-half kilograms of Semtex high-explosive and petrol. The judge said: "It was a device which was of truly devastating capacity for causing death, serious injury and destruction. On explosion, the debris or shrapnel could be thrown hundreds of metres.''
The petrol meant that "there could have been a fireball and those in range would have no chance of escaping harm, injury or possible death".
Fryers smiled and gave his lawyers a thumbs-up sign before being led to the cells. He told the court: "When I came to this country I believed what I was doing was right, and still believe that."
he hoped that the peace process would work "and will be just and lasting and free from British interference".
Jack, from Sauchie, near Alloa, central Scotland, showed no emotion as he was sentenced.
The judge told Jack that he had gone into bombing campaign "with your eyes wide open".
The two men made unlikely terrorists. Fryers, a former night club bouncer, was indiscreet, while Jack had a string of convictions for drunken brawls.
From the very first day, the terrorists, based in a council flat in Sauchie, were under surveillance by MI5 and police from three forces.
The authorities believed the IRA's aim was to penetrate the "ring of steel" set up to protect the City of London. They had enough explosives for a further six bombs.
Fryers was arrested in London with the first bomb. Jack, who hid bomb-making equipment in a wood, was arrested shortly afterwards.
MI5 officers gave evidence anonymously for days during the trial, unlike police officers.
There are concerns that if MI5 were to move into other operations, such as fighting drug dealers or organised crime, more details would be hidden from the public, which could compromise the legal system and lead to miscarriages of justice.
Michael Mansfield QC, defending Jack, criticised the use of screens and the anonymity granted to MI5 witnesses during legal arguments, claiming that the prejudicial effect was "substantial".
He said: "It is getting very close to a trial in camera."