So often has the power-sharing arrangement in Northern Ireland teetered on the brink of collapse that the temptation is to turn a blind eye until the threat is either averted or comes to pass. The temptation is especially great at a time when the calls on the attention of politicians and public on the mainland are so pressing. From migration to the Chinese stock markets to the meltdown in the Labour party, there is quite enough to be going on with as the new political term begins. Northern Ireland is always with us, is it not?

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Yet there is more than enough reason to sit up and take notice this time around. The Northern Ireland executive could fall apart very soon indeed – as soon as the Ulster Unionist party acts on its decision to leave the government.

What happens then is anyone’s guess. An agreement designed to provide contingencies for just such an eventuality was blocked by Sinn Fein and never ratified. The stark choice would appear to be between early elections for the Northern Ireland Assembly (which might solve nothing) and the reimposition of direct rule from London for the first time in a decade.

No wonder then that the latest fracas is being treated by Westminster as rather more than a little local difficulty. David Cameron is making space in his diary today to meet Northern Ireland’s first minister, Peter Robinson, while the – conspicuously unimpressive – Northern Ireland secretary, Theresa Villiers, will meet the Irish Republic’s foreign affairs minister, Charlie Flanagan, who describes the current situation as “grave”. The imminent general election in the republic is a politically complicating factor.

What prompted the UUP’s (unanimous) vote to withdraw from the executive is a dispute that is both new and old – which is why it is so potentially destructive. In contention is how far the provisional IRA still exists, 17 years after the Good Friday agreement and seven years after its council was declared “no longer operational or functional”. The argument burst into the open after the chief constable of Northern Ireland, George Hamilton, said that provisional IRA members were involved in the murder last month of an ex-IRA man, Kevin McGuigan. In fact, the chief constable was more careful in what he said than he is often given credit for. He specifically excluded claims that McGuigan had been killed on the order of any high command. But what he said, and the fact of his position, gave succour to those – not just in the UUP – who had never trusted the IRA to disband or to put its weapons “beyond use”.

Facebook Twitter Pinterest ‘What prompted the UUP’s (unanimous) vote to withdraw from the executive is a dispute that is both new and old – which is why it is so potentially destructive. In contention is how far the provisional IRA still exists, 17 years after the Good Friday agreement.’ Photograph: Peter Morrison/AP

In retrospect, he would have been well advised to leave the IRA and the politics out of it, unrealistic though that might be. A murder, after all, is a murder.

The various attempts, including by Villiers, to limit the damage only kept the chief constable’s remarks in the public eye – and the fallout has dominated the news on both sides of the Irish border. Peter Robinson and Gerry Adams between them made heroic efforts to call a halt, to no avail. It is not just the power-sharing executive that is now threatened but the elaborate diplomatic compromise represented by the Good Friday agreement.

For the awkward truth, being illustrated for the umpteenth time, is that politics in Northern Ireland is singularly resistant to the ambiguity that oils the wheels of diplomacy. The Good Friday agreement was hard-won and enshrined that necessary ambiguity. Perhaps some genuinely believed that the IRA and the Provos had both forsworn their armed struggle, but many did not. The agreement stuck, to the extent it did, because enough people embraced the fiction of disarmament, in the hope that it would in time become reality.

Continuing disputes about flags, marches and the past show how little room there is in Northern Ireland, even now, for constructive ambiguity.

But the absence of violence, for the most part, is accepted as a vast improvement on the Troubles, even if the republicans and loyalists remain as separate, ideologically and geographically, as ever. This is why the reappearance of the IRA question, with its undertones of weapons and command structures, is such a thoroughly unwelcome, and dangerous, development.

The existence – or not – of the IRA, however, is not all that is pulling the Northern Ireland executive apart. For months now, ministers have been split over welfare reform, with the looming prospect of bankruptcy, should costs not be cut. And one of the reasons why the dispute has dragged on for so long, is because there is a fall-back position.

Even if Northern Ireland overspends disastrously, the likelihood is that it will be bailed out one way or another by London. And something similar applies to the dispute over the IRA. Westminster stands anxiously in the wings, ready to do whatever it takes to prevent a return to armed conflict.

What we have here is devolution under a continuing paternalistic and apprehensive eye. Neither one thing nor another, it amounts to an invitation to irresponsibility. London has imposed direct rule on Northern Ireland four times since 1998, and could do so – in fact, if not in name – a fifth. Even the democratic credentials of the power-sharing government are questionable, as the format leaves no real room for opposition.

Whether because successive UK governments have not allowed it, or because so many of its politicians – with some honourable exceptions – seem immune to compromise, Northern Ireland has not grown up. Whether the analogy is the umbilical cord or the apron strings, it remains unhealthily attached. The result, in security terms, is our very own “frozen” conflict.

Consider two comparisons. You could look across to Scotland and suggest that Northern Ireland takes its fate into its own hands, as the SNP government has done there. The flaw in this argument is that Scotland remains subsidised and will not, so long as it remains in the UK, run the full risk of bankruptcy, even if it heinously overspends. That eventuality has not yet arisen, but it could.

The other is with the Irish Republic. Difficult though recent times have been, the Irish government stands or falls at the behest of Irish voters. If it overspends, its resort is to the EU or to international lenders. It, and it alone, is responsible for internal order and external security.

There is nothing like national self-determination for instilling a sense of grown-up responsibility. UK-style devolution, which resists not just break-up, but genuine federalism, risks leaving its constituent parts in an adolescent Never Never Land.