Sam McBride: May and the DUP face a dilemma – but so does Sinn Fein

On leaving the Conservative Party, they complained about the influence of the ERG and the DUP on the prime minister.

The Westminster distaste for aspects of the DUP – some real, some imagined – has been obvious since the signing of the confidence and supply agreement in mid-2017.

But by further depleting Mrs May’s already slender working majority, the most immediate impact of those MPs’ departure is to make the DUP yet more influential, thus potentially leading to increased angst on the Tory benches and perhaps more resignations which would make the DUP more powerful still.

That leverage for Nigel Dodds and his colleagues would, however, come to an abrupt end if the trickle of resignations became a flood.

If even the DUP’s 10 MPs are insufficient to keep Mrs May in Downing Street, then a general election is inevitable unless Jeremy Corbyn can suddenly persuade enough MPs to endorse him – and he is facing the same problems as Mrs May with resignations from his own benches.

But although the DUP is now at the centre of a high-stakes power game, it is not the only Northern Ireland party in that position.

Sinn Fein, despite not taking its seats in Westminster, is increasingly going to find itself in the spotlight.

Although the party’s argument for abstentionism has been confused by contradictory utterances from various senior figures, its core has remained constant and is two-fold: Westminster is a foreign parliament which has no right to rule over a part of Ireland, and Westminster is irrelevant because Northern Ireland’s 18 MPs have no real power.

The latter of those arguments has since the 2017 election been wrong – and that is being increasingly exposed by the narrowing Commons arithmetic.

If Sinn Fein could shape Brexit but chooses not to do so, that is a huge decision for Mary Lou McDonald – not just in Northern Ireland, but in the Republic.