There may be some way to go, but today we moved much closer to a British climbdown on the question of the Irish border after Brexit. And this will turn an acrimonious debate on its head. So far, we’ve been talking about the implications of Brexit for Ireland. Now we have to talk about the implications of Ireland for Brexit.

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It is not just that Britain’s weakness in its negotiations with the European Union has been made even more starkly clear. On the three issues on which “sufficient progress” had to be made – people, money and Ireland – Britain seems likely to suffer a hat-trick of defeats.

Its concessions in the talks on the border issue are not yet official, and may seem more abstract and less visceral than its retreats on the divorce bill and the rights of EU citizens in the UK; but they may prove to be much more fundamental and much more problematic for the whole Brexit project. There is a sense here of the return of the repressed: the Brexiters pretended Ireland did not exist; now it has come back to haunt their grand schemes.

It is hard to think of a more boring phrase than the key one in the draft agreement between the UK and the EU: “continued regulatory alignment” on the island of Ireland. Yet within this technocratic formulation there lie all the things that Brexit’s true believers should have been worrying about if they were not so busy patronising Ireland with cheery and meaningless reassurances about not returning to the borders of the past. It means, in essence, that Northern Ireland will at a minimum have to behave as if it is still in the customs union after Brexit.

This may or not be explicitly stated in the final deal, but it is now the unavoidable destination of this process. You can’t have regulatory alignment if you’re in different customs regimes. Thus, even before the trade talks begin, the Brexit promised by Theresa May – a clean break for all the UK from the customs union and single market – is almost certainly off the table. Hard Brexiters always feared that the Irish border might be a Trojan horse entering the citadel of their pure certitude. The wooden horse is now well and truly inside the gates, even if it has not yet been opened.

The first thing to note about the draft agreement is that Northern Ireland is, after all, not Yorkshire or Sussex. The line from the British government has, until now, been that of its allies in the Democratic Unionist party, whose leader, Arlene Foster, continues to insist that “Northern Ireland must leave the EU on the same terms as the rest of the United Kingdom”. The DUP, from its own point of view, is right to be deeply alarmed by what is unfolding – if this is indeed the deal, the DUP will have suffered a historic defeat.

What is now on the table is a concession that, whatever else happens, Northern Ireland will in effect stay in the same customs and regulatory regime as the Republic of Ireland, which is of course the same as that of the EU as a whole. So Brexit doesn’t quite mean Brexit after all – it has become complex and ambiguous. The apparent success of the Irish insistence on getting real commitments to the avoidance of a hard border means Northern Ireland will exit Europe through a different door.

And this raises, for people in Britain, a rather more explosive question. It is no longer whether Northern Ireland will leave the EU on the same terms as the rest of the UK. It is whether the rest of the UK will now leave the EU on the same terms as Northern Ireland. Given what seems to have been conceded, there is only one way for Northern Ireland not to have a special status – and that is for all the UK to remain in the customs union.

The hard Brexiters like to see themselves also as hard unionists. But these two positions have just become radically incompatible. There are just two possible outcomes. If Northern Ireland in effect stays in the customs union and Britain leaves, then there will have to be an internal UK customs border, checking goods moving between Northern Ireland and British ports. This undoubtedly weakens the union.

But the only way to avoid this is for the UK as a whole to stay in the customs union – which of course the true believers don’t want either.

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For the moment, this hard choice will be evaded with rhetorical promises that the eventual trade deal between Britain and the EU will be so frictionless and painless that borders won’t matter anyway – except, of course, for all the migrants who have to be kept out.

But if we did not know already that this is fantasy, we know it now. The climbdown we are seeing on all three of the preliminary negotiating issues surely ends the illusions of all but the most deluded fanatics about Britain’s real position in the Brexit process. It is not in a position to make demands – certainly not demands that the EU destroy its whole raison d’etre by allowing a member state to leave the single market but still enjoy all its advantages.

It was always stupid to turn the border issue into a face-off between mighty Britain and little Ireland. But that’s how the hard Brexiters and their Tory press allies chose to construe it.

Having done so, they might now ask themselves: if, for the first time in 800 years, Ireland is proving to be in a much stronger political position than Britain, what does that say about what Brexit is doing to Britain’s strength? It is being forced to accept what it claimed to be unacceptable, not because Ireland has suddenly become a global superpower but because it has the unflinching support of EU member states, the European parliament, and the EU negotiating team. There might be a lesson in there somewhere for a country facing a future without the allies it has long taken for granted.

• Fintan O’Toole is a columnist with the Irish Times