Last Friday, a paddlesteamer named Oliver Cromwell started to sink on its journey from north Wales to Northern Ireland, delighting both fans of pregnant political metaphors and enemies of the Butcher of Drogheda alike. On the same day, two-thirds of the votes placed in Irish ballot boxes committed the Irish government to changing the law on access to abortion. The surprise announcement that the repeal side had won left the six counties in the north in a peculiar position: now out of step with both the Republic and the rest of the United Kingdom, on both abortion and gay rights.

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Theresa May’s alliance with the DUP has always been a deeply unwise balancing act, and now it appears to be taking on water in the Irish Sea while all the other political parties watch on from dry land. It could easily be the case that the Conservatives simply didn’t know what they were getting themselves into: the morning after the 2017 general election, a senior columnist from another paper joked to me and a Tory MP, all waiting to go on air to share our supposed political expertise, “I guess we’ve all got to Google the DUP now.” People quickly picked up on its antediluvian views on gay rights, abortion and creationism, but very little was said about its history of virulent sectarianism. Even as the UK voted in that election, the DUP MP Emma Little-Pengelly was facing criticism for perceived inaction over the erection of flags celebrating the loyalist terror group Ulster Volunteer Force outside a mixed housing development. Five months later, four Catholic families fled the street in fear of their lives.

The warning signs that the Tories had no idea what they were doing were there from the outset: in a stunning display of both arrogance and stupidity, they announced they had reached a deal with the DUP before negotiations had begun, assuming a party with 10 MPs would acquiesce rather than face the embarrassment of correcting them. But the embarrassment was all Theresa May’s, as well the DUP knew, as they forced the Conservatives to step back up to the lectern and admit their error. The next error was assuming that if the Tories flew over for the weekend, the DUP would meet them on a Sunday. If Tory special advisers failed to pick up the most basic knowledge about the working habits of Free Presbyterians on the Sabbath, it’s little surprise they took so long to broker a deal. Unbelievably, the Conservatives hopped on a flight to Belfast assuming it would be a breeze to broker a deal with a party that sat through all of the negotiations leading to the Good Friday agreement and still opposed it.

A year on, we have had the abortion referendum landslide in Ireland, to which May reacted by tweeting her congratulations to the yes campaigners and adding that the vote was an “impressive show of democracy which delivered a clear and unambiguous result”.

But even before the official result was declared on Saturday, the leaders of Sinn Féin, Michelle O’Neill and Mary Lou McDonald, were pictured holding aloft a hand-drawn cardboard sign stating “The North is next”. Labour’s shadow minister for women and equalities, Dawn Butler, called the fact Northern Irish women were still subject to Victorian-era UK law – denied abortion and forced to travel elsewhere in the UK to get the care they need, or face prosecution and imprisonment at home – “an injustice”.

Keeping gay marriage and abortion illegal has long been made possible by a combination of the Republic opposing both, Sinn Féin maintaining some social conservatism and the rest of the UK largely ignoring the north. Now all of those supports have evaporated: Sinn Féin campaigned to repeal the eighth amendment, and is pushing the British government to legislate on same-sex marriage in Northern Ireland. Both access to abortion and LGBT marriage rights will be enshrined in law in the Republic, and the DUP is scrutinised now more than it has been since the heyday of Ian Paisley.

But May is trapped by her deal with the DUP: she can’t budge on abortion rights for Northern Irish women without risking the collapse of her rickety coalition. And remaining deaf on this subject will increasingly be seen as a moral betrayal, not just to people in Northern Ireland but to everyone in the UK, if Northern Ireland is hypocritically treated as a special case so May can save her government.

This hypocrisy runs through the ongoing Brexit negotiations too. New research published by academics at Queen’s University Belfast on the attitudes of people in Northern Ireland show that the majority of people back same-sex marriage rights: 62% of people overall, with 75% of Catholics and 50.5% of Protestants backing mooted legislation.

The one point in particular that should alarm senior Conservatives and Unionists massively: on the question of reunification with Ireland, Catholics are largely ambivalent. But were Brexit to deliver a hard border, removing Northern Ireland from the EU customs union and single market, Catholic support for reunification rises from 42% to 58%. The DUP has maintained that it will not accept Northern Ireland being treated as a special case, which means it will not accept a “soft” Brexit that would keep the north remaining in the single market and customs union as the rest of the UK leaves. But this is precisely the fudge the majority of Northern Irish people want and, given the DUP’s position, is the issue that could swing public opinion towards political reunification with the Republic.

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For the Conservatives, confronting the DUP over gay marriage and abortion rights will be a dress rehearsal for the battle to come. Like a drunk at a party, they eventually have to be confronted and asked to leave – and they won’t be happy about it. One senior Sinn Féin politician told me recently that Arlene Foster and the current DUP stable were, unlike Peter Robinson and DUP founder Paisley, utterly opposed to bowing to any middle ground, as the ongoing Stormont shutdown shows.

But May will have to put her foot down on Brexit, to Foster and her own party. It’s either that, or accept that, while David Cameron will be remembered as the prime minister who took the United Kingdom out of Europe, thanks to her shonky and ill thought-out deal with the DUP, she is likely to be feted by Irish republicans as the British prime minister who handed back the six counties.

• Dawn Foster is a Guardian columnist