“CURRY my yoghurt can coca coalyer.” That piece of doggerel may have collapsed the Northern Irish peace agreement and thrown another spanner in the Brexit works.

It was uttered by a DUP MP in 2014, making fun of the Irish language phrase for “thank you Mr Speaker”, which is used by Sinn Fein members of the defunct Northern Ireland Assembly. That, and similar linguistic abuse, encouraged Sinn Fein and the SDLP to demand a law protecting the Irish language. It was the DUP's rejection of this last week that led to the collapse of the negotiations to restore the Stormont power-sharing arrangement. The DUP say they won't accept any Irish Language Act unless it also covers Ulster Scots, arguing that more people speak Polish in Northern Ireland than speak Irish, a claim that is disputed by most authorities on the Northern Irish Gaeltacht.

The protection of the Irish language may seem a bizarre issue to cause the collapse of devolution and plunge the province back into pre-1998 constitutional limbo. No-one had suggested making it compulsory or the official language of the North. In Scotland, Gaelic is legally protected, as in Wales, even though it is spoken regularly by only by a handful of the population. Here, there has been some grumbling about Gaelic place names and road signs popping up in areas where the language was rarely spoken, but not even Scotland in Union has called for the restoration of direct rule by Westminster. The Democratic Unionist leader, Arlene Foster, called for exactly that last week.

Indeed, in what appeared to be a calculated snub to Theresa May, she waited until the PM had actually arrived in Belfast last week to sink the agreement – leaving May bereft and looking faintly ridiculous. The DUP insist that it was Sinn Fein who did the dirty. But it is hard to believe that Number Ten would have allowed the Prime Minister to come all the way to Belfast only to leave empty-handed, had the Northern Ireland office not been given some sort of nod from the DUP that a deal was likely. Sinn Fein's new leader, Mary Lou McDonald, insists that a draft agreement was in place. She says it included an Ulster Scots Act and a Respecting Language and Diversity Act.

This row is the most dramatic sign yet that the politics of Ireland are being destabilised by Brexit. In the recent past, such an issue as the Irish language bill would have have been resolved with good will in the spirit of the Good Friday Agreement. But the row has become a cypher for the wider question of the status of the North after Brexit. As with its veto of the Brexit phase one agreement last year, the Democratic Unionists are using a crisis to force the Government’s hand. They want the UK to take over the direct running of the province.

The DUP are worried that the UK is giving up on Northern Ireland in its desperation to get a deal with the European Union. They fear, with some justification, that the UK is prepared to allow a de facto reunification of Ireland if that is the only way to avoid a hard border emerging on the Irish mainland. The UK has given repeated assurances to the DUP that this is not its intention. But the way the issue is being fudged has led to fears in the province that there will effectively be a border around the whole of the island: meaning an east-west border in the Irish Sea, rather than a north-south one on the mainland. The embattled Ulster Protestants would rather see a hard border in Ireland than lose their status as part of the United Kingdom.

But here's the problem. Northern Ireland has already lost its status as an indivisible part of the United Kingdom. During the Northern Ireland peace process, the UK gave up its territorial claim to the province. It said it had “no selfish strategic or economic interest” in Northern Ireland and would accept the reunification of Ireland if the people of the North voted for it. The UK Government repealed the 1920 Government of Ireland Act to make this clear. The Irish Republic made a similar change to its constitution, abandoning its territorial claim. It now speaks only of “a desire for the peaceful political unification of the island subject to the consent of the peoples of Northern Ireland and Ireland”. This effectively turned the North into a stateless province – but it kept the peace for 20 years.

The reason the Belfast Agreement worked was because it was under the auspices of, and in the constitutional context of, the European Union, and incorporated the European Convention on Human Rights. This specifically addressed the language question, stressing “the importance of respect, understanding and tolerance in relation to linguistic diversity”. Under the so-called Strand Two section of the agreement, cross-border institutions were set up for “implementation of EU polices and programmes”. Because the constitutional question was raised to a higher level, both communities in Northern Ireland felt secure. The DUP no longer feared incorporation into the republic and the extinction of its culture. Republicans felt they could lay down their arms and allow demographics to bring about the eventual reunification of Ireland.

For many in the nationalist community in Northern Ireland, being part of the EU meant that, to all intents and purposes, Ireland had been reunified. There was no border to worry about. Free movement was guaranteed. Joint membership of the European single market meant that the economies of the North and the South were becoming indistinguishable. The Irish question became an issue of culture and identity rather than nationhood and citizenship. Irish republicanism faded away, and loyalists, with their antique sectarian passions, lost the patience of their own people. The DUP and Sinn Fein entered government together. Brexit has blown this out of the water.

Nationalists now face the possibility of a hard border reappearing, losing their joint citizenship and the EU guarantees of their civil rights. The Protestant DUP has revived fears that Ulster will be incorporated by stealth. The Republic has secured a guarantee from the EU that there will be no hard border and there seems no obvious way of doing this without keeping the North in the single market. The idea of full regulatory alignment implies that the North will be in a different relationship to the EU than the rest of the UK. This could mean that the North ceases to be “as much a part of the UK as Finchley”, as Margaret Thatcher used to describe the constitutional relationship of Ulster to Britain. Direct rule is an attempt to forestall this.

Of course it is too early to start talking about a collapse of the peace accords. As Gerry Adams said when he stood down as president of Sinn Fein, people have become too used to peace to contemplate any return to violence. But the Brexit process and the threat of a border has created a profound insecurity in the province. The collapse of devolution and power-sharing is a classic case of unintended consequences, and no-one knows where the province goes from here. It is nearly 20 years since the paramilitaries started decommissioning their weapons. Let's hope the guns continue to rust in peace.