There is good cause, based not just on a yearning for peace but on sound political reasoning, to believe that the bombers who inflicted such damage have also inflicted huge damage on their own organisation and, indeed, on all those who would resort to terror.
During the course of the Troubles relatives of many victims have voiced the hope that their deaths would have some meaning and would help prevent the deaths of others. Tragically that hope has not been realised: the killings have gone on.
But Omagh is different. It will be a turning point because its terrible results have at a stroke swept away so much of the ambiguity about violence which lurked in so many Irish minds, nationalist and Unionist. It has cemented an emerging coalition of democrats and those edging towards democracy; it has isolated the violent as never before.
The so-called Real IRA is about to face huge pressure from police north and south of the border. It has no credible political wing to make its case, no coherent theory to expound, no perceptible public support on which to fall back. There will be raids and arrests, and within weeks there will be legislation specifically tailored to crush it.
But the sheer weight of public revulsion generated by Omagh poses if anything even greater challenges for the terrorists. Anyone even reputed to be associated with them is going to be ostracised.
Last Friday, for example, Bernadette Sands-McKevitt, the voice of the 32 County Sovereignty Committee which has been linked to the Real IRA, could not open her print shop in Dundalk: the locks had been changed. Rallies are being held in Dundalk to dissociate the town from what it regards as deeply undesirable elements.
In former days groups like the IRA had a network of sympathisers to give them comfort during the hard times, but the Real IRA is too small, too unrepresentative and too new to have developed such support mechanisms. Its members are pure militarists with no grasp of the politics of it all.
The May referendum on the Good Friday Agreement demonstrated conclusively that nearly all republicans, north and south, endorsed the accord and were prepared to give the political path a chance. The Real IRA simply disregarded this, its mouthpieces dismissing the referendums as illegal and undemocratic. It maintained, ludicrously, that it had a mandate deriving from the last all-island elections of, wait for it, 1918. It drew on that dangerous little piece of old republican dogma which holds, with breathtaking arrogance, that the people have no right to be wrong.
That nonsense has been swept away by an Omagh fatality list that reads like a roll call in miniature of the Troubles. The town has a proportion of Catholics to Protestants of roughly 60-40. On a sectarian head-count the dead consisted of approximately half Catholics and half Protestants.
The bombers killed young, old and middle-aged, fathers, mothers, sons, daughters and grannies. They killed a prominent local member of the Ulster Unionist party. They killed people from the backbone of the Gaelic Athletic Association, a group which represents an important unifying ingredient of small-town and rural Irish nationalist life.
They killed unborn twins, bright students, cheery shop assistants and many young people. They killed three children from the Irish Republic who were up north on a day trip. Everyone they killed was a civilian. The toll of death was both extraordinarily high and extraordinarily comprehensive.
After previous atrocities such as the IRA bomb which killed 11 Protestants in nearby Enniskillen in 1987, the terrorists were able to ride out the storms of outrage. After some days nationalists would start to feel the authorities were intent on milking such deaths for propaganda purposes. Then troops would perhaps shoot somebody in controversial circumstances, providing the IRA with extenuating martyrom and valuable cloud-cover.
That will not happen this time, for a number of reasons. One is the sheer scale of that death-list, and its indiscriminate nature, which speak of either murderous intent or sheer incompetence.
Most mainstream republicans, it should be noted, do not believe the Real IRA wished to kill all those people, regarding the attack as a catastrophic foul-up rather than deliberate mass murder. But even with that belief, they do not go on to absolve the Real IRA of blame. So this time, possibly for the first time ever, mainstream republicans find themselves in the same camp as the rest of the body politic, reacting with revulsion rather than scrambling to find ways to excuse and mitigate.
That is an illustration of the strength of the Good Friday Agreement, endorsed as it was by nearly all northern parties and all political groupings in the south. The agreement won 71 per cent support in the north, a vote which has been described as momentous but not conclusive.
In one sense the agreement and its endorsement can sound like little more than a combination of standard politics and a mathematical exercise, but the fact is that it has developed a mysterious appeal and power far in excess of its individual components. Some might have thought that Omagh might have blown the fledgling accord apart: instead it has fortified it.
On the day they buried Avril Monaghan, who died with her mother, her daughter and her unborn twins, a family statement said: "She was looking forward to bringing up her family in a peaceful society following the Good Friday Agreement and the recent referendum." Even on that day of unbearable grief the bereaved held out the agreement as a source of hope.
The accord is going to face many more crises, for its functioning depends on the successful establishment of a series of interlocking institutions. Some time in the next few months, for example, an executive is to be formed to run the new Belfast assembly.
Since membership is to be determined on the basis of assembly seats, Sinn Fein is looking forward to occupying two places on the executive - in effect two ministries in government. Nobody is sure whether David Trimble's Ulster Unionist Party, which is deeply divided, will accept that.
Mr Trimble is asking for republican concessions, such as significant moves on IRA arms decommissioning or a declaration that the mainstream IRA's war is over. Nobody knows whether Sinn Fein will be willing or able to deliver on any of this.
The arguments on those issues continued even last week, but significantly they were conducted not in shrill, hysterical terms but in a more subdued fashion than previously. The scale of Omagh's horror made other problems pale into insignificance.
The stature of Mr Adams and Mr Trimble rose in their respective camps. The republican impressed some Unionists with his unequivocal condemnation of the bombing; the Unionist was warmly applauded by the congregation when he attended the funeral of three Catholic children in the Irish Republic.
The bombing has dissolved none of the political problems facing Northern Ireland but it has had a bonding effect. Many have taken from it the moral that the only alternative to the Good Friday Agreement is more violence. It is now pretty well inconceivable that any of the important elements supporting the agreement could now think of walking away from it.
It has been a horrendous experience but it has also been a shared one, the bombers inflicting a parity of grief on the two traditions. All the signs are that it has had a devastating effect on the Real IRA and on some of the other violent splinter groups.
This does not mean that it can be said that the violence is over. But the wave of revulsion has been so utterly overpowering that it is difficult to imagine any armed group, republican or loyalist, finding it possible to wage a sustained campaign of violence again.
If so, the attack on Omagh may prove a watershed of historic significance. Nothing can bring back the dead, but that thought may in future provide some little solace and consolation to the survivors and the bereaved in their terrible grief and pain.