It will be ironic if Tony Blair's elusive legacy is located, finally, at Stormont. A terrorist campaign more efficient and lethal than anything in Islamic fundamentalism is brought to a peaceful, negotiated conclusion. A movement founded on the rejection of British values is reconciled, finally, with British institutions. When he comes to write his memoirs, Blair might stop to wonder how he did it.

He did not do it alone, of course. Gerry Adams may never knowingly risk a vote he could lose, but the decision on Sunday by Sinn Fein's Dublin Ard Fheis (conference) to co-operate with the Police Service of Northern Ireland (PSNI) is a triumph for political evolution. As so often before, yesterday's terrorist has become today's statesman.

In the space of 20 years, Irish Republicanism has recognised the legitimacy of the Dail, discarded the gun, negotiated with Unionism, embraced coalition with Ian Paisley and accepted a central tenet of democracy: the rule of law. And, no mere detail, Adams is still alive.

Dr Paisley will claim his share of the credit unaided. He will present a lifetime spent saying no as a necessary preparation for final, gracious assent. He will boast that because he has never deviated in his distaste for Roman Catholicism he has brought Catholic Nationalists to heel. Then he will deviate, and deviate some more. When the blackly comic photograph is taken at last, more than a few supporters of the Democratic Unionist Party will avert their eyes.

After all, if - always have a supply of qualifications to hand where the north of Ireland is concerned - Sinn Fein's resolve holds, there will be elections on March 7. If - and never say ever, in these matters - the DUP fails to find a new reason to delay the horror, a power-sharing executive will be formed on March 26. The doctor will become First Minister.

Yet there at his side in the photograph, fresh from the cut-and- paste of history's Photoshop, will be Martin McGuinness, IRA commander and proud deputy leader of a devolved government under the British Crown. Upwards of 3000 will have died over 30-odd years for the sake of surrealism, belated pragmatism and an administrative arrangement.

Paisley and McGuinness boast nothing in the way of joint policies, of course. No-one knows if they can work together for more than five minutes. For example, Sinn Fein's leadership claimed on Sunday that the point of accepting the PSNI was to clean up policing, not least after the recent revelations of Loyalist death squads sponsored by the Special Branch. Paisley's Unionist constituents, with memories of 300 police officers killed at IRA hands, will take a different view.

That's just the start. It is, however, a real start after innumerable false dawns. The Provos, most of them, have been on ceasefire since 1997; the Good Friday Agreement was signed as long ago as April, 1998. In July of 2005, the IRA's Army Council declared that its campaign to end British rule was at an end. Yet still the haggling went on. Adams cajoled and wooed the Republican movement. Paisley blustered and bullied, yet prepared to accept the inevitable. Controversies came and went, but the essential facts survived.

The strategy being pursued by Adams will not drive Britain from the island of Ireland any time soon: Republican purists are right about that. Their problem is that "armed struggle" was, if anything, even less profitable. Many volunteers died, and for what? Another heave? Another bombing "spectacular" more staggering than the last? Long before many of their followers, Adams and McGuinness realised that Republicanism was becoming nihilism by another name. It had to stop.

When it stopped, Paisley inherited the problem. Mere bigotry is sustainable when you decline to participate in the political process. When you aspire to power, you aspire to democratic respectability. You cannot simply refuse to co-operate with "them" just because of who they are, not in the real world, not if they have a mandate, not if they insist on passing every test of political legitimacy. Adams and McGuinness have deprived the doctor of his excuses.

Yesterday, the best that the DUP could manage was the insistence that Sinn Fein's words must be matched by "actions on the ground" and an "end to criminality". But otherwise - teeth may have been gritted - the "move towards government" would be accepted "positively". Having defeated the Ulster Unionist Party (UUP) in the struggle for the Protestant vote, the DUP now bears the burden of leadership.

Some among them won't care for that. Let us be blunt: they don't like Catholics. Some Republicans, equally, are liable to detect another historic betrayal of the old cause and return to the malevolent fringes. Let's be equally frank: if you exist only to "get the Brits out", anything less is liable to sound disappointing, and deeply infuriating. Dissidents on both sides could still provide the main protagonists with familiar excuses to renege.

The fact remains that Paisley, Adams and McGuinness, the erstwhile extremists, are the only game in town. The UUP and the theoretically-Nationalist SDLP have only a middle-class appeal. Their memberships are ageing and shrinking. Other minority parties, meanwhile, have no real traction. The IRA is on ceasefire; Loyalist gangs are imploding and returning to gangsterism. Politics, to the relief of the people of the province, is beginning to resemble something normal, ordinary and dull.

Sinn Fein could not have hoped to enter government while boycotting the institutions of justice: such gestures no longer apply, and no longer make sense. The DUP, equally, could not hope to "speak for Ulster" while gerrymandering the facts of political life in Northern Ireland: Catholics, in increasing numbers, tend to vote. So the old enemies are locked together, each convinced that the upper hand is theirs.

Just in case London and Dublin have failed to notice, Adams is pursuing a two-state solution. The Republic will go to the polls this summer. A newly-respectable and acceptable Sinn Fein might hope to win 10 to a dozen seats in the Dail. That would not turn the world upside down, but it could give the party a place in a coalition government reaching out, economically and politically - the process has already begun - to Stormont. And who would be the junior partner in that northern administration?

Paisley, for his part, will be gone, soon enough, mourned by some. In the meantime, his successors grow accustomed to the political realities the old man spent a lifetime despising and, latterly, accommodating. The Irish state is modern, successful, European and just down the road. Mainland Britain, oceans apart, hopes merely to hear less of its province in Ireland in years to come, but, meanwhile, stakes no inalienable claim. The old contours become visible once more.

The IRA did not win Sinn Fein a place in Stormont, but a place in Stormont may do more than the Provos ever did, one day, to reunite Ireland. As though in mirror image, Paisley's lifetime of opposition and rejection may yet conclude with two communities bound more closely together than ever before. Two ironies for the price of one society.

In the meantime, expect another minor crisis, another war of words, another round of brinksmanship. Expect some meaningless violence, too. Nothing happens overnight in Northern Ireland. But never entertain the fiction that nothing ever happens.