On the assumption that he has learned from some past mistakes (and the evidence is that he has), Peter Mandelson's appointment to Stormont may be counted good news on two counts: for Mr Blair's government and, more crucially, for Northern Ireland.

This is to imply no particular comment on the speed with which Mr Blair has restored his favourite son. The Tories were quick on the attack last night. And Downing Street will be scanning normally friendly papers this morning for any judgment that the Prime Minister still makes too light of conflicts of interest in his ministerial ranks.

Nor is it to join any rush to denigrate Dr Mowlam. For all the falling-out which has marked her protracted departure, history will acknowledge her critical contribution to the peace process.

This was particularly so in the period before the last general election when the Major government simply ran out of steam, and it fell to Dr Mowlam to persuade sceptical nationalists and republicans that a Blair government would do serious business over Northern Ireland.

That said, Dr Mowlam's departure was overdue. She had steadily lost the confidence and trust of the unionist community. And as their attacks on her became ever more personal, somehow she allowed the impression to grow that the dislike was mutual. As one close observer put it last night, "The body language let her down."

Mr Mandelson arrives in Belfast this morning as the Mitchell review enters its decisive phase, and as the British government prepares for a serious discussion with the unionist community about the one issue above all which retains the capacity to destroy the Belfast Agreement, the future of policing in Northern Ireland.

There is no reason to believe Dr Mowlam would have been other than repulsed by the manifestations of what Mr Trimble routinely describes as "the mafia society" there. However, she showed no real understanding of the depth of ordinary unionist fears that the peace process is, ultimately, about the empowerment of paramilitaries.

It is perfectly arguable that the unionist political class has maligned Chris Patten, is mistaken and misguided in its fixation with titles and symbols, and is missing the prize offered by Patten - to have substantial police powers devolved to the Northern Ireland Assembly.

But ministers have shown little awareness of the deep-seated suspicion among law-abiding citizens that the "logic" of the process somehow points to paramilitaries in police uniforms and the ceding of control to those who have long enforced their rule of terror in estates and towns across Northern Ireland.

Fairly or unfairly, had Dr Mowlam sworn on a stack of Bibles seven days a week that it would not be so, unionists were simply not disposed to believe her.

There is no reason to suppose Mr Mandelson has any interest in allowing the principles of policing to be corrupted. Indeed, those who know him say his every instinct would be to the contrary. Persuading unionists of his bona fides - while keeping faith with nationalists and republicans, and Dublin, on police reform - will be central to his task of bolstering the political process.

And the process can only be bolstered by this appointment. From at least his days as a producer on LWT's Weekend World, Peter Mandelson has had a developed interest in the Northern Ireland issue. One old friend suspects "he is probably instinctively pro a united Ireland", adding, "but then we know that he carries his ideological baggage lightly."

Certainly long-time Mandelson watchers are agreed it is hard to imagine any other member of the Blair government who might be expected so speedily to grasp the politics and the detail, the nuances and the sensitivities on all sides of this most complex brief.

It is also impossible to imagine any way in which Mr Blair could better have signalled his continued seriousness about the peace process. The former "sultan of spin" yesterday expressed the hope that his appointment would be seen as evidence of the British government's "unwavering commitment" to Northern Ireland. And seldom, if ever, can Mr Mandelson have expected that his words should be taken at face value.

There is no doubt about his ability to do the job. There is no doubt either that he will carry himself as Secretary of State with the full authority and confidence of the Prime Minister.

It remains to be seen if Mr Mandelson can completely cure Northern Ireland leaders of their historical tendency to run to Downing Street at any sign of difficulty. But Mr Trimble in particular has got what he asked for, and what he said he wanted, a Northern Ireland Secretary capable of carrying the load. There was some speculation last night that the Mandelson appointment signals a readiness by Mr Blair to get tough with the Northern Ireland parties. That is probably not how it should be read.

However, we can expect less tolerance henceforth of the recurring unionist notion of a Northern Ireland Secretary operating as a loose cannon, beyond prime ministerial control. For, as one shrewd Irish observer remarked last night, "Peter Mandelson will be even closer to Tony Blair's ear than David Trimble."

OF THOSE past mistakes? One Mandelson admirer says: "He'll have to bring some of his new-found humility with him. He's capable of being arrogant, somewhat haughty, blowing hot and cold with people. He'd do well to curb that." And, on the evidence of the way in which Mr Mandelson has managed his rehabilitation, he believes he will.

The new Secretary of State will also know that the ways of the spin doctor will not play well at Stormont, that unionists, nationalists and republicans alike prefer plain talking and straight dealing.

So, just 10 months after his enforced resignation, Peter Mandelson is back in the big picture; with a real job to do, no guarantee of success, hope revived that he will eventually make it to the Foreign Office, and with the opportunity to explode some of those Mandelson myths.

Northern Ireland itself, however, will explode the biggest myth of all, of Mandelson as magician. For as Seamus Mallon sharply observed, the fundamentals are not altered by a change in personnel. And if the Belfast Agreement is to be made to work, the big choices and the big challenges are still down to the parties there.