There were these two Irishmen on a bus... Tasteless, certainly, but it's not such a bad starting point after this week's IRA explosion on a London double-decker. For jokes deal in stereotypes and what the past week has revealed is the extent to which such thinking continues to bedevil the relationships which have imploded into this latest desperate stalemate.

Both of the young Irishmen who had apparently been travelling on the 171 to Aldwych were the kind, their families and neighbours insisted, who could not possibly have been responsible for the bomb on the bus. But one of them was - Edward O'Brien, the 21-year-old from Co Wexford, who rang home every month and whose last reported words were on the phone to his mother a few hours before he died: "See you ma, I love you."

Ed O'Brien had come to Britain in search of work after a long period of disgruntled unemployment in his home town of Gorey. As a youth, he had never talked about politics. A relative said: "The only thing he lived for was hurling and drink. He was a very shy, gullible kid who wouldn't even raise his finger to anyone."

A neighbour said: "I kept telling myself he must have been a passenger who was in the wrong place at the wrong time."

In London, O'Brien was just another unremarkable young Irish emigrant living among thousands. With no criminal record and no known Republican links, he would have fitted perfectly the "clean security profile" required for the IRA's most critical work on the British mainland.

The IRA handbook stipulates "Your prime duty is to remain unknown to the enemy forces and the public at large." O'Brien came very close to meeting that expectation: his family did not even have an address for him when he died, and his identity remained uncertain for almost two days after the bombing.

But on Tuesday lunchtime, at 17 Allenwood Drive, Gorey, O'Brien's parents, Myles and Margo, opened their door to two local police, who said: "Prepare for the worst."

The "worst" was that the man killed in the Aldwych bus bomb was almost certainly their son, and that he had been an active IRA member implementing the most lethal form of the Republican "physical force" philosophy.

The Irish police, the Gardai, believe there are just six active Republicans living in this part of Wexford. Today, the only visible sign of the nationalist cause is a memorial to insurgents who marched through Gorey en route to join the 1798 "year of the French" rebellion in Wexford.

Father Ford, the local priest, is highly sceptical of suggestions that Ed O'Brien could have formed any Republican links in Gorey. "This is not an especially Republican place," he says. Far from being a home-grown IRA member sent abroad as a "sleeper", Father Ford is convinced O'Brien was recruited in London. "A young guy going to a strange city - he's lonely; he's also aware of his ethnic background. It becomes a breeding ground and the IRA have become experts in spotting them."

And for 24 hours this week, the world assumed the IRA must have spotted and recruited Brendan Woolhead, the other Irishman rescued from the bus bombing.

Woolhead was badly injured in the explosion. Within hours, the police told the press that it looked like an "own goal": it would be an extraordinary coincidence if he was not involved along with O'Brien. Policemen in bullet- proof vests armed with Heckler and Koch machine pistols were posted at the end of his hospital bed.

The media reports the next day accepted the police account. The Daily Mail called Woolhead a "suspected IRA bomber". The Sun, which referred to O'Brien as "the fiend", called Woolhead "his accomplice" and "the injured Provo".

But the proximity of the two men was simply an extraordinary coincidence. Next day, Woolhead, who it turned out was not on the bus but walking by it, had become in a Sun exclusive the "innocent Irishman" and the "hapless Brendan".

There is surely a lesson to be learnt from all this. From stereotypes, it is a small step to creating "suspect groupings", after which demonisation can follow and, for a tiny minority, the violence that then becomes a self-fulfilling prophesy.

There is nothing new about the derogatory stereotyping of the Irish. In the 1180s, the Norman propagandist Gerald of Wales was already branding them a barbarous, filthy and irresponsible people who "think that the greatest pleasure is not to work". During the 19th century, Punch was known for its caricatures of the Irishman as Paddy O'Caliban, a Simian brute with bended knees and a shillelagh, barely civilised and prone to emotional violence. The music-hall image of the feckless, stupid, priest- ridden, drunken, combative and ceaselessly talkative Irishman has persisted since.

Even today, in more subtle form, the dynamic persists. Nowhere is that more true than in the way terrorists are depicted. "Ever since Joseph Conrad and Dostoevsky, an erroneous assumption has been made that paramilitaries are abnormal, psychologically unhinged or inherently unstable individuals," says Brendan O'Leary of the London School of Economics. "In fact, all the studies show that by every other measure, they are as sane and normal as the population around them."

Demonising them is an easy way to discount their arguments. Yet if terrorists love their mothers and are liked by their fellows, that makes them more rather than less dangerous opponents.

According to a survey conducted by Professor James O'Connell of Bradford University, 80 per cent of people think that violence of republican and loyalist paramilitaries had harmed the standing of the Irish who live here with the rest of the British public.

Now, for the quarter of a million Irish-born people in London, the sense of being ostracised and singled out for public contempt looms again. "Thank God for supermarkets," one Irish woman said recently. "I can do my shopping there without having to open my mouth". With a Travelcard, a young Irishman might get on a bus the same way.