Success? A deal appears to have been struck on the Irish border, paving the way for the Brexit talks to move from legacy issues – the question of the United Kingdom’s outstanding liabilities, what happens to British citizens in the rest of the EU and European citizens in the United Kingdom, and the treatment of the Irish border – to the shape of the future relationship.

According to RTE and Philippe Lamberts, a Belgian Green MEP who has been shown a draft agreement, the British government has agreed a “special situation” for Northern Ireland.

The question of what happens to the Irish border after Brexit is difficult but not complex. Since 1993, when the customs union came into being in its current form, the United Kingdom and the Republic of Ireland have shared a customs and regulatory orbit, which has allowed them to maintain an open border. Once the United Kingdom leaves the European Union, if the British government wishes to also diverge as far as customs and regulation goes, there will be customs checks, either at the existing border between the Republic and Northern Ireland, or in the Irish Sea. The first is unacceptable to the Irish government, the second is untenable for the DUP, on whom Theresa May relies to remain in office, and indeed to parts of the Conservative Party as well.

The third option – the United Kingdom remains inside the customs union and within the regulatory orbit of the European Union – is acceptable to a majority of MPs in the House of Commons, but it is unacceptable to a big enough minority of Conservative MPs that it is hard to see how Theresa May would be able to avoid a no confidence vote, though she could probably survive it.

What has been agreed appears to be a form of words that can broadly, be read in different ways by different people. For the Irish government, it’s a guarantee that there will be no hard border between the Republic and Northern Ireland. But for the DUP, that control over these powers are devolved to the legislature in Stormont means that they wield a veto over any actual regulatory alignment, at least in theory.

It basically comes down to who is actually aligning with you: does the agreement mean that come the crunch, Northern Ireland will align with the Republic of Ireland even if the rest of the United Kingdom doesn’t, or that that Northern Ireland will have the option to do so, or that the United Kingdom as a whole will remain within the regulatory orbit of the United Kingdom? It’s not clear. That fudge means that all sides can probably agree to move forward with talks on the next stage of agreement.

There are a couple of things to watch out for as far as what happens next. The first is that it is difficult to see how power-sharing in Northern Ireland will resume for the foreseeable if the final outcome is one where the right to diverge is concentrated at Stormont, not Westminster. Why? Because power-sharing at Stormont requires the largest party from each community – currently, the DUP and Sinn Féin – to agree a joint programme. This fudge means that the government at Stormont will be given the choice: to use its regulatory devolution to cleave closer to the Republic of Ireland or to the United Kingdom. That’s a poison pill as far as any accord between the two groups at Stormont goes.

But a more definitive steer on regulatory convergence risks tipping the Conservatives out of office as the DUP cannot accept an arrangement that weakens Northern Ireland’s connections with the United Kingdom while strengthening their ties with the Republic.

What about the third option? Well, that’s where things get a little more complicated. If Theresa May committed to that tomorrow, the Conservative party would implode and so would her premiership. But if she can get to the end of the process and agree a fairly soft exit from the EU, she is guaranteed a large enough chunk of votes from the Labour Party to protect her from any Conservative rebellion, for a variety of reasons: devoted pro-Remain Labour MPs won’t vote to allow a catastrophic no deal exit, Labour MPs with a heavy constituency Leave vote won’t want to be seen to be resisting the “will of the people”, and so on. Even if the majority of Labour MPs vote against, May ought to be able to count on a big enough chunk of them to see off any rebellion.

So the interesting question is whether this fudge can hold together long enough to get May to the end of the line, or if it will collapse in short order.