Three days ago, the Democratic Unionist party (DUP) seemed to hold the fate of Europe in its hands. Ever since, it has been forced to stand gloomily in the wings watching while Boris Johnson struts in the limelight, backslapping those “blackmailing burghers of Brussels”, as Sammy Wilson MP, the DUP’s Brexit spokesman called the EU leaders last year. Whatever happens in the vote for the Brexit deal in parliament this weekend, the special relationship with the Tories that enabled the DUP to act like old-style one party rule had returned to Northern Ireland is over.

Did the DUP really think it could have and hold Lord Snooty just for the price of a dinner in a Ballymena hotel?

Just last month Johnson shambled into a DUP gathering at the Conservative party conference in Manchester to joyous shouts of “Boris! Boris! Boris!” In a speech almost drowned out by stomping and cheering, he declared that as he looked out at the “sea of eager happy faces tonight supporting our precious union I know it is in good shape and that we unionists are in good heart”. DUP leader Arlene Foster almost curtsied. A year ago Johnson was the belle of the ball at the DUP conference in Belfast. By rejecting Theresa May’s deal, they had saved the UK and saved Northern Ireland from being turned into a “semi-colony of the EU” he declared. He even promised them the bridge to Scotland for which they have been clamouring. Everyone else saw the bridge was a postmodern joke. Johnson could hardly stop laughing.

Well, more fool the DUP. Sympathy for its plight now will be scant. The party shafted the pro-remain majority of voters in Northern Ireland when it sold its 10 parliamentary votes to the Tories in exchange for sole access to power and influence in Westminster. It was also a move that showed mutual contempt for the Good Friday agreement, which requires the British to exercise “rigorous impartiality” in dealings with Northern Ireland and its parties.

May learned the hard way that the DUP’s loyalty is only to itself. How could the DUP then have failed to see that the same is true of the Little Englanders? Ian Paisley Jr, in between luxury, all-expenses-paid holidays in exotic countries with dodgy human rights records, hosted various Tory grandees at fundraisers in his constituency. Did the DUP really think it could have and hold Lord Snooty just for the price of a dinner in a Ballymena hotel?

It may well have done. When talks fail, money has been found to talk with the DUP: when it was wrongly reported on Wednesday that the party was onboard for Johnson’s deal, there was widespread cynical speculation about how many millions the prime minister had promised them. The party that used to be most strongly associated with puritanical biblical rectitude is now notorious for its apparent devotion to Mammon.

Play Video 1:17 MPs say Boris Johnson’s Brexit deal is far worse than Theresa May’s – video

On Thursday, the party’s deputy leader Nigel Dodds accused Johnson of being too hasty in agreeing a deal. (The three painful years of Brexit negotiations represent not even the blink of an eye in the timescale of the biblically minded.) The fact of the matter was, Dodds said: “If he held his nerve and held out he would, of course, have got better concessions.”

Well, here is a parable: a few years ago, a DUP minister in Stormont was told by his private secretary that a poultry company based in Northern Ireland wished to give him a free turkey for Christmas. The minister was in the middle of making decisions that would have a bearing on the future profits of the company – he was told that the turkey could be small, medium or large. The minister asked his private secretary to phone the company and ask them if they had something bigger.

That story is included in a remarkable book just published in Ireland. If the black clouds of the past week had just a sliver of a silver lining for the DUP it is that the sudden escalation of Brexit negotiations took some attention away from author Sam McBride’s revelations about the DUP’s true attitude to the UK. The book, Burned, is about the renewable heating incentive (RHI) scandal in Northern Ireland. It reveals that the party was, it seems, quite happy to keep the so-called “cash for ash” incentive going in its beloved province for as long as it could milk the British treasury to pay for it. Panic only set in when it emerged that Stormont had to pay for the overspend. McBride compared the RHI legislation introduced in the UK in 2011 with the legislation brought into Northern Ireland the following year by Foster, then minister for enterprise, trade and investment. They were almost identical – except that 107 words had been removed from the Northern Ireland version: the ones that dealt with cost controls.

McBride is a journalist of integrity, and the political editor of the News Letter, a pro-union paper. He speaks of “reckless profligacy – or worse”. Sinn Féin was a willing partner in this “economic nationalism”, but for Irish republicans the British were for centuries the imperial enemy and old habits die hard. Senior DUP figures and their relations were among those piling into a scheme that offered business and non-domestic users a refund from the treasury of £1.60 for every £1 of fuel used, in a bid to get them to switch to renewable energy sources. The scheme was to be “grandfathered” for 20 years. A Brazil-based multinational poultry company was set ultimately to reap the highest benefits. After years of costly delay, and damage to the environment, the scheme was shut down – eventually the Northern Irish assembly collapsed in the wake of the scandal, among other divisive issues. In writing his book, McBride sought and was refused interviews with Foster and other key DUP figures.

The DUP knows it will have to carry the burden of Brexit | Katy Hayward Read more

Foster insisted to Sir Patrick Coghlin’s inquiry into the scandal that she did not recognise there was a “perverse incentive” to burn fuel at the heart of the scheme, despite having been told of it by a whistleblower. Plenty did spot it. Boilers blazed in empty churches with open windows. Poultry catchers complained they were sweating so much they had to change their clothes during shifts. As comedian Tim McGarry put it, there were farmers in Northern Ireland putting on oven gloves to open the door of their sheds.

The report of the RHI inquiry is imminent, and will not endear the DUP to the British taxpayer. Fresh from its humiliation over the Brexit deal, on Monday the party will make a last-ditch – and bound to fail – bid to stop a Westminster bill from legalising abortion and same-sex marriage in Northern Ireland. With perhaps unfortunate timing, it is also about to face its party conference. The DUP is going to have to break it to the faithful that there will be no bridge to Scotland.

• Susan McKay is an Irish writer and journalist