Earlier this year I was talking to the director Jim Sheridan about his most recent film, The Boxer. It's a kind of political thriller set during the first IRA ceasefire and centring on a former IRA man played by Daniel Day-Lewis. He gets out of prison, tries to start a new life and sets up a non-sectarian boxing club.
Around him are two gangs: the mainstream IRA, moving towards an end to violence, and a middle-aged man and wife, who represent the fierce, unappeasable strain of fundamentalist republican terrorism.
At the end of the film Day-Lewis, who has fallen foul of the diehards and defied their reign of terror, is about to be killed by the diehard IRA man. Suddenly, like something halfway between an avenging angel and a guardian angel, the other IRA, the one that's making peace, appears. The Bad Provo is killed by the Good Provos, and as the credits roll something like a normal life can begin.
What struck me in talking to Jim Sheridan about this was his statement that he hadn't quite realised until he had made the film what this last scene was really saying.
Translated from the indirect language of a popular movie into the direct language of politics, it was saying there is nothing to be done with those whose violence cannot be appeased but to kill them. And, of course, that those who would do the killing would be their former comrades.
A few years ago, too, the BBC made a television film based on a script by the republican sympathiser Ronan Bennett. Even before the first IRA ceasefire, it tried to envisage how the "war" would end. There was a Michael Collins-like Provo leader played by Brendan Gleeson, manoeuvring his way out of the armed struggle.
And, in the end, there was a scene where his opponents within the movement were lured to a barn somewhere and simply massacred.
Watching scenes like these on screen, you feel good. This is, no doubt, largely because they are part and parcel of the genre. We've all grown up on thrillers and cowboy movies where the bad guys get rubbed out in the last scene and evil is purged from the world.
But it is also because, at a more local and political level, we've all had this movie in our heads. There's something both neat and clean about the notion that the last bit of dirty business will be done by those who already have blood on their hands. Those of us whose hands are clean won't have to be sullied. The hard men will do it for us.
It's not, moreover, pure fantasy. We know from 20th-century Irish history that the most dangerous enemy for a gunman is a former comrade-in-arms. From Michael Collins and the Civil War, to de Valera's crushing of the IRA when he was Taoiseach in the 1940s and 1950s, to the feuds between Provos and Officials and within the INLA, the various offshoots of the republican movement have long been still more ruthless in the suppression of internal dissent and disloyalty, and the punishment of those from whom they have split, than in the pursuit of their "official" enemies.
Yet, for all that, the fantasy that the IRA would clean up its own rubbish was always a dangerous illusion. It fed the hallucination that ruthless violence could ever have a legitimate function in the circumstances of contemporary Irish politics. It is only in movies that a few moments of violent catharsis are followed by the onset of peaceful normality.
In real life murder, all murder, leaves a void that craves more and more bodies. And, in any case, the whole point of a democratic settlement is that no political problem, even that of what to do with the pathological killers left over after the conflict, can be solved by self-appointed enforcers.
This is not to say that Sinn Fein and the IRA do not have a special responsibility to bear in the common task of breaking the so-called "Real IRA" and its allies once and for all. On the contrary, for all their undoubtedly sincere expressions of horror and disgust, Gerry Adams and Martin McGuinness must be haunted by the knowledge that the people who bombed Omagh are probably the same people who bombed towns and cities all over Ireland and Britain during the last 25 years.
If Omagh is appalling, horrifying and disgusting to them, why did they react to Birmingham and Teebane Cross, to Enniskillen and Shankill Road, to Warrington and Mullaghmore, with excuses and platitudes, with orations and commemorations - not for the innocent victims, but for the killers? The only acceptable answer to that question will be the decommissioning of weapons, a declaration that the war is over and honest and full assistance in the search for the perpetrators of Omagh.
BUT the fact remains that the IRA and its apologists have no moral right to condemn, let alone wipe out, the so-called "Real IRA". That burden falls on democratic governments. And it does so at a time when Irish democracy itself is morally vulnerable. It has been caught out in the open, in a no-man's-land between the demands of law and decency on the one hand and the exigencies of conflict-resolution on the other.
Just at the moment when those who are guilty of past atrocities are being given a kind of indulgence for their actions by early prison releases, Omagh reminds us of the deep human and moral imperative never to indulge for one moment the sick stupidity of those for whom any political cause justifies the mass slaughter of children, men and women.
No fantasy gang of repentant killers is going to save us from the terrible contradictions we face by getting rid of its old comrades. No one can get us out of an immensely painful contradiction. We're saying to the victims of previous atrocities that their tormentors, if they have been caught, will be released early, and, if they have not been caught, they will not now be pursued.
We're also saying to Omagh's victims that the perpetrators of the atrocity will be ruthlessly pursued and punished as they deserve. We're trying at one and the same time to bring mass murderers back into the fold of civilised society and to push mass murder once and for all outside the fold of society.
It makes no sense, and yet there is no alternative. It is the kind of vicious, awful contradiction that is created by a vicious, awful situation. On an island where someone tears the flesh off babies on a main street on a Saturday afternoon, there are never going to be clear, easy moral choices. All we can do is remember again and again the sorrow of obliterated lives and the pain of shattered bodies and work forward from the absolute certainty that whatever ends up in that hellish place is as wrong as it is possible to be.