Perhaps it is a problem of language. The dictionary lacks a necessary word: UKish. Northern Ireland is not in Britain but it is in the UK. The porous boundary that divides it from the rest of Ireland is not, strictly speaking, a British frontier. So it is called “the Irish border”, making it, for the Brexiters, someone else’s problem.

The terrain where a post-Brexit UK meets the remaining 27-member EU bloc is, as the miserable Tory leadership debate shows yet again, somewhere over there. For Boris Johnson and Jeremy Hunt, its troubles are, as Neville Chamberlain might put it, “a quarrel in a faraway country between people of whom we know nothing”. They might have to know more about it if only we could call it what it is: the UKish border.

Hunt and Johnson both agree that a no-deal Brexit must be kept alive as a serious proposition. Both also agree that once in Downing Street they will reopen negotiations with the EU with the primary aim of ditching from the withdrawal agreement the so-called Irish backstop, which is also, of course, the UKish backstop. Hunt puts this in more emollient terms than Johnson, but makes up for this weakness by promising to include on his negotiating team representatives of the famously emollient Democratic Unionist party, which does not represent most voters in Northern Ireland.

All of this is so drearily familiar (this hobbyhorse comes round and round on the non-stop Brexit carousel) that it is hard to remember how surreal it is. It is weird not just because the backstop was designed around British demands; not just because Hunt and Johnson were in the cabinet when it was negotiated; not just because they both voted for it in parliament; and not just because the EU has repeated, over and over, that, in the words of the European council in January: “The backstop is part of the withdrawal agreement and the withdrawal agreement is not open for renegotiation.” That should be enough to be going on with, but there is an even deeper absurdity.

On 4 April last, the German chancellor, Angela Merkel, flew to Dublin. This was the moment the Brexiters had been waiting for: Britain had held its nerve and now Merkel (in their worldview the puppet master of Europe) was going to deliver the bad news to the taoiseach, Leo Varadkar – Germany had done its best to protect the Irish peace process and prevent a hard border on the island, but that game was up. Realpolitik was now the order of the day: bye-bye backstop.

That was the dreamworld. In reality, Merkel met Varadkar but made no demands for change to the withdrawal agreement. And then she spent time talking and listening to people who live and work and run businesses on both sides of the border. When she emerged from the meeting, she said: “For 34 years I lived behind the Iron Curtain so I know only too well what it means once borders vanish, once walls fall.” She understood the border question more deeply, and had much more emotional investment in it, than the government that is actually responsible for it – the British cabinet. This is what is so bizarre – the German chancellor has spent more time listening to the UK citizens who live on the border than either Johnson or Hunt ever have. She understands what it’s like when a wall falls; they don’t.

No facts must be allowed to disturb the tranquillity of this blissfully wilful ignorance

All of this, of course, passed the Brexiters by. It belongs to the world of inconvenient truth. Oliver Norgrove, who was a staffer on the official Leave campaign, recently recalled in the Irish Times an awkward moment about a month before the referendum of June 2016. A request came in from the BBC’s Newsnight. Would the Leave campaign send a representative to debate the effects of Brexit on the Irish border? “Nobody in the office,” recalled Norgrove, “was keen to take up the request, with even our more polished and experienced media performers rejecting the opportunity on the grounds that they simply lacked real knowledge of the issue. I remember quite vividly the feeling of unease and discomfort about the prospect of us talking about something we just didn’t feel needed addressing.”

And, as both Hunt and Johnson have shown, no facts must be allowed to disturb the tranquillity of this blissfully wilful ignorance. Here, nonetheless, are three of them. First, the backstop in its final and troublesome form is not an Irish or European invention. The original version (agreed in December 2017) affected Northern Ireland only and did not tie the rest of the UK to the customs union. It was changed because the British side, under pressure from the DUP, insisted.

Second, the EU Withdrawal Act, passed at Westminster in 2018, states that nothing in it may “diminish any form of north-south co-operation provided for by the Belfast agreement”. This is UK law – and an official mapping exercise identifies 142 policy areas of north-south co-operation, 51 relating to the operation of the north-south ministerial council established under the agreement. A no-deal exit or a ditching of the backstop would break that law.

Third, the backstop does not preclude the “alternative arrangements” of “abundant, abundant technical fixes” that Johnson promises. On the contrary, if he actually believed in that promise, the backstop is irrelevant – it comes into play only in the absence of a workable alternative.

But these are mere facts and thus as meaningless as the most obscure fact of all: that Northern Ireland is part of the UK, a state whose very shape is defined by its frontiers. But that raises the even bigger issue of the union itself and what Brexit will do to it. We can be sure that neither Hunt nor Johnson wants to ask that awkward UKish question.

• Fintan O’Toole is a columnist with the Irish Times and author of Heroic Failure: Brexit and the Politics of Pain.