The culmination of the referendum campaign was the BBC’s live Great Debate from Wembley on the evening of 21 June 2016. It lasted for two hours. After an hour and a half, someone finally raised the question of Britain’s obligations under the Belfast Agreement of 1998 (often called the Good Friday Agreement) that brought an end to the longest and most vicious internal conflict in the history of the United Kingdom. Frances O’Grady, general secretary of the TUC, spoke passionately in a tone of pleading desperation: “Many trade unionists in Britain and Ireland worked together for many years to support the peace process in Northern Ireland and it took a lot of hard work. And we’ve supported the Good Friday Agreement ever since … The Irish prime minister has said that if we come out of the EU, there will have to be border controls and, let me tell you, the way that is seen in Belfast and Derry, I worry for our future.”

It fell to Boris Johnson to reply to O’Grady on behalf of the Leave campaign. “I remember vividly,” he said, “when the EU was given the task of trying to sort out the tragedy in the Balkans…” For those who had suspected that, for most of the Brexiters, Ireland might as well be Montenegro, here was literal confirmation. Johnson spoke for two minutes. He did not address O’Grady’s point at all. He did not manage even the most facile of cliches about the Belfast Agreement or the benefits of peace. He did, admittedly, raise the question of violence on the streets: “I do worry about our security on the streets of this city.” This city was, of course, London. The UK cities that O’Grady had mentioned – Belfast and Derry – were neither here nor there.

Facebook Twitter Pinterest In 1985, British soldiers in Northern Ireland make checks on vehicles near the Irish border. The significance of border controls for the peoples of Northern Ireland and Ireland seems lost on Brexiters. Photograph: Alain Le Garsmeur/Getty

Or perhaps it would be more accurate to say that they are very much “there” – a foreign place barely glimpsed even in peripheral vision. Johnson has been utterly consistent in this. In his big Brexit speech earlier this month, delivered as foreign secretary, he said about the Belfast Agreement exactly what he said in June 2016: not a word. There is some irony in the way he has implicitly adopted the great vernacular slogan of people in Northern Ireland during the Troubles: whatever you say, say nothing.

The consequences of Brexit for the Belfast Agreement have always been, for the most ardent Leavers, the thing that must not be seen or spoken. And in the way of such things, the whole Brexit process has been haunted by the return of the repressed. The EU has refused to pretend that there is no Irish problem. It has not gone along with the British government’s airy assurances that “there will be no return to the borders of the past” on the island of Ireland because everything is going to be frictionless, seamless and marvellous. The EU insisted, in the first phase of negotiations, on turning these airy assurances into solid commitments.

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This week, Brussels will publish its draft withdrawal agreement, a legally binding text under which the UK will, in effect, commit itself to keeping Northern Ireland in the single market and customs union, unless a future free trade deal or a magical technological solution manages somehow to avoid a hard border. It will also bind both parties to recognising in all negotiations the “paramount importance”, as Theresa May wrote in her Article 50 letter, of the Belfast Agreement.

This has enormous repercussions. The sleeping beauties on the Tory right, for whom ignorance of Ireland was Brexit bliss, are finally waking up to these implications. A fog of denial and self-delusion is beginning to clear and they can at last see what should have been obvious all along: you can have a hard Brexit or you can have the Belfast Agreement but you can’t have both. And it is increasingly clear which choice they want Britain to make: throw the dead weight of the peace process overboard so that the Brexit balloon may soar into the blue skies of its triumphant future.

Facebook Twitter Pinterest In 2000, five years before being elected as an MP, Michael Gove wrote a pamphlet that portrayed the peace process as a capitulation to the IRA. Photograph: Andy Rain/EPA

The Tory right has never been fond of the Belfast Agreement, even though it was John Major who (with considerable courage) prepared the way for it by explicitly removing any British claim over Northern Ireland other than the democratic wishes of its people. In The Price of Peace, Michael Gove’s pamphlet for the rightwing Centre for Policy Studies, published in 2000, he characterised the entire peace process as nothing more than a capitulation to the IRA which he likened to the appeasement of the Nazis in the 1930s. But this hostility lapsed over time into mere indifference – until, very late in the day, Brexit’s true believers began to understand that they have an Irish problem.

Hence, as the EU prepares to publish its draft legal text, what seems like a concerted campaign to clear the greatest moral and political obstacle from the path to Brexit glory. The Belfast Agreement must be made not to matter. The failure of attempts to re-establish a power-sharing executive in Belfast opens up the opportunity to say that the whole 1998 deal is no longer worth bothering about.

Owen Paterson MP (@OwenPaterson) The collapse of power-sharing in Northern Ireland shows the Good Friday Agreement has outlived its use https://t.co/Qc72wysJpr @RuthDE via @Telegraph. @NIOgov

And so the former Northern Ireland secretary, Owen Paterson, tweeted that “The collapse of power-sharing in Northern Ireland shows the Good Friday Agreement has outlived its use”. Labour’s staunchest Brexiter, Kate Hoey, follows up by declaring the agreement “unsustainable”. The leading Tory intellectual, Daniel Hannan, in his Daily Telegraph column, dismisses the agreement as nothing more than “a bribe to two sets of hardliners” in Northern Ireland. He claims, rather astonishingly, that it did not bring peace because Northern Ireland was already at peace: “The Belfast Agreement was a consequence, not a cause, of the end of terrorism.” And to crown the campaign, Jacob Rees-Mogg, also writing in the Telegraph, announces that this whole Irish business does not really exist – it is an “imaginary problem” caused by the Irish government.

The Belfast Agreement is not holy writ and the collapse of the power-sharing talks between Sinn Féin and the Democratic Unionist Party does, indeed, point to the problematic nature of one part of it – the internal arrangements for the governance of Northern Ireland. If the context were less febrile, the 20th anniversary in April of the agreement would be an apt opportunity for a thoughtful review of those structures.

Facebook Twitter Pinterest Leading Brexiters seem not to understand the deal Theresa May, pictured, signed last December, which she said ‘must be upheld in all circumstances’. Photograph: Stefan Rousseau/PA

But let’s not kid ourselves. These attacks on the agreement are not coming from a sudden interest in the minutiae of devolved government in Montenegro – sorry, Northern Ireland. They are all about Brexit. The real subject here is not a deal made in Belfast in 1998. It is the deal Theresa May signed in Brussels in December to conclude the first phase of the divorce negotiations. Bizarre as it may seem, many of the leading Brexiters seem not to have understood that deal or the radical implications of the commitments May made in it, commitments that, she concurred, “must be upheld in all circumstances, irrespective of the nature of any future agreement between the European Union and United Kingdom”.

It is worth recalling that when May did this deal in December, the first instinct of the Brexiters was to characterise all the language in it about Ireland as just more meaningless verbiage to stop the Irish from whining on. The Brexit secretary David Davis told Andrew Marr: “This was a statement of intent more than anything else. Much more … than it was a legally enforceable thing.” This was entirely untrue – as we will see this week, the deal will be turned into a legally binding text – and Davis quickly reversed himself and claimed to have been misunderstood. But his claim revealed a deeper mindset: an inability to grasp the underlying reality that the Belfast Agreement sets very tight limits on the UK’s final arrangements with the EU.

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The problem for the Brexiters is that, in December, May signed up to two different things, apparently without grasping the connection between them. The first of these is that the Belfast Agreement effectively demands that there be no hard border between Northern Ireland and the Republic. Thus, whatever else happens – even if there is an ultra-hard, no-deal Brexit – the British will “maintain full alignment with those rules of the internal market and the customs union which, now or in the future, support North-South co-operation, the all-island economy and the protection of the 1998 Agreement”.

This is a big commitment in itself, but it could be implemented in a way that had relatively little impact on post-Brexit Britain by, for example, creating customs borders between the two islands of Britain and Ireland rather than within Ireland. Except that this kind of arrangement is anathema to the DUP, on whom May depends for her Commons majority. So, in desperate need of a deal, she also gave the DUP what it wanted: an equal commitment that there will be “no new regulatory barriers” between Northern Ireland and Britain. The mathematics of this are quite simple. Northern Ireland has to remain fully aligned with the Republic, which is to say with the customs union and single market. Britain has to remain aligned to Northern Ireland. So Britain has to remain aligned to the customs union and single market. Exit hard Brexit pursued by the Belfast Agreement.

Facebook Twitter Pinterest Parliament buildings at Stormont in Belfast. To achieve the desired vision of Brexit, Theresa May would have to renege on her promise to protect the Belfast Agreement. Photograph: Clodagh Kilcoyne/Reuters

One way to deal with this is heroic denial. The Brexit minister Robin Walker told MPs on Thursday that the text agreed in December “is not talking about full alignment” – even though “full alignment” is exactly what it says. The other is to dig out the root of the whole problem, the Belfast Agreement itself. In a sense, this is the more honest approach, not least because it implicitly acknowledges that the driving force in Brexit is a specifically English nationalism for which the rest of the UK is ultimately dispensible.

But even then there is a problem for the zealots. Their whole vision of a glorious post-Brexit future is based on Britain’s ability to do great trade deals and be a trusted partner on the world stage. Yet to get there they now have to start by tearing up two of the most important international deals Britain has signed in its recent history, both of them legally binding. They have to renege on the pact May signed in December, that placed the Belfast Agreement at the centre of the Brexit process. And they have to pull out of the agreement – which, contrary to what they seem to believe, is not an internal British deal but a binding international treaty between two sovereign governments registered with the United Nations and effectively underpinned by both the United States and the EU.

They would have Britain stand before the world, knee-deep in shredded treaties, and say, “Sign here, trust us!” One can but wish them the best of British luck with that one.

Fintan O’Toole is a columnist with The Irish Times