To know what a thing is you have to know where it begins and ends, and where its edges are. And the awkward fact is that the United Kingdom begins and ends in the blurry, meandering, impossibly ambiguous border that snakes its weary way around the six north-eastern counties of Ireland. This is a fact, not just of Irish history, but of British history.

If you listen to some of the more anguished Brexiters, you might get the impression that the Irish border is a devious plot against the people of the neighbouring island, a snare invented by the Irish to trap Britain in European vassalage. In reality, it was created at Westminster, through the Government of Ireland Act, which became law in May 1921. It is, first and foremost, a British problem and a British responsibility. At the heart of the current impasse over Brexit is a shocking refusal to accept that responsibility.

The ‘precious union' stuff is a gaudy orange garment borrowed from the DUP to cover the most nakedly obvious attitude

Brexit has always had a large dose of phoney populism – it is an elite project for extreme globalisation wrapped up as a popular revolt against globalisation. But it also has an equally large dose of phoney unionism – it is an English national rebellion wrapped in the union flag. Among its many contradictions, perhaps the one in which the gap between rhetoric and reality yawns most widely is this one.

On the one hand, all the evidence is that neither those who voted for Brexit nor those who are seeking to drive it to the hardest possible conclusion give a good goddamn for the union, and in particular for Northern Ireland’s place within it. On the other, they are pretending to be willing to walk away from the Brexit negotiations at any cost if the bloody Irish and the scheming Europeans don’t stop making proposals for dealing with the border question that would – horror of horrors – undermine what Theresa May calls “our precious union”.

Thus the need for a guarantee that there will be no border infrastructure in Ireland is not a recognition of Britain’s duties to its own citizens. It is, in the words of the letter written to May this week by Boris Johnson, David Davis, Jacob Rees-Mogg and others, “a trap being set by the EU which it is vital we do not fall into”. In this self-pitying narrative, the basic facts of modern British history become a foreign conspiracy against “our status as a sovereign nation state”.

These people want a no-deal Brexit, and they are using the integrity of the union as the excuse for it: the people of England, Scotland and Wales supposedly prefer to suffer a no-deal catastrophe than agree to checks on goods going between Stranraer and Larne. Such bureaucratic procedures – utterly invisible to the vast majority of the population – would allegedly amount to an act of sacrilege against the most sacred thing in the heart of every Briton: the integrity of the union. They would suggest that Northern Ireland, contrary to a famous claim by Margaret Thatcher, is a little less British than her constituency of Finchley. And that can never be tolerated.

Play Video 1:04 Michel Barnier says Irish border issue could sink Brexit talks - video

This has all the conviction of a brothel-keeper taking umbrage at the very mention of sex. At every stage of the Brexit process we have seen complete indifference to the fate of Northern Ireland. The “precious union” stuff is a gaudy orange garment borrowed from the DUP to cover the most nakedly obvious attitude: this Irish border stuff concerns a faraway people of whom we know nothing and care less. It is not our concern but merely a distraction being used by the EU Brexit negotiator, Michel Barnier, to deny us the perfect Brexit we deserve and demand.

The evidence for this is threefold. First, in the 2016 referendum campaign, the leave side consistently refused to address the problems of Northern Ireland, the Good Friday agreement of 1998, or the border. The best that could be got out of any of them was a double fantasy. The border would not be a problem because the Republic of Ireland (never really recognised as an independent country) would just have to follow the UK out of the EU and return to the fold of what could again be called (political correctness be damned) the British Isles. And/or the EU would have to give post-Brexit Britain a fabulous trade deal – the easiest ever negotiated – that would ensure “frictionless” trade anyway, so again the border did not have to be considered.

Second, we know from the overwhelming survey evidence that English people who voted leave in 2016 or supported the Tories in 2017 felt little responsibility for Northern Ireland and did not care about the peace process. The most telling measure of commitment to a political union is whether or not you are content for taxes from the richest part to be used to subsidise services in the poorest. In the recently released Future of England study, just 25% of leave voters and 29% of people who voted Conservative in 2017 agreed with the proposition: “Revenue raised from taxpayers in England should also be distributed to Northern Ireland to help Northern Irish public services.” (Tellingly, the only people still willing to subsidise Northern Ireland are the remainers.)

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And when asked whether “the unravelling of the peace process in Northern Ireland” is a “price worth paying” for Brexit that allows them to “take back control”, fully 83% of leave voters and 73% of Conservative voters agree that it is. This is not, surely, mere mindless cruelty – it expresses a deep belief that Northern Ireland is not “us”, that what happens “over there” is not our responsibility.

Third, since May triggered article 50, we have had, on the crucial border question, nothing but bluster and magical thinking. The prime minister has signed up twice, in December and in March, to the so-called backstop to ensure that Northern Ireland remains fully aligned with the Republic to avoid a hard border. The rest of us are entitled to assume that she knew what she was doing. And yet here we still are, with May trying to wriggle away from these commitments. Why? Because they were predicated on the emergence of some as yet uninvented technology for invisible border control – Johnson, while foreign secretary, seriously proposed that the 500km (310-mile) Irish border could be controlled like the congestion charge in London.

So spare us at least the hand-on-heart, tear-in-the-eye sentimentality – if this is what a precious union amounts to, it is hard to imagine what a throwaway union would look like. If the Brexiteers are really looking for a last ditch in which to fight for the purity of their cause, it should be somewhere they actually care about. They mentally ditched Northern Ireland a long time ago.

• Fintan O’Toole is a columist at the Irish Times and author of Heroic Failure: Brexit and the Politics of Pain (published in November 2018)