Arlene Foster, an Ulsterwoman with the keys to power

Four months ago, Arlene Foster’s political longevity was being measured in weeks. Besieged by a public spending scandal and suffering from flu, she was miserable and cranky at the launch of her Democratic Unionist party’s manifesto.

Electoral comeuppance duly arrived. The DUP, the political voice of Ulster unionism, was just 1,200 votes away from being pushed aside as Northern Ireland’s largest political faction in the devolved assembly, by its arch-rivals in Sinn Féin. “The notion of a permanent unionist majority in the north [of Ireland] has been demolished,” snorted Gerry Adams, the Irish republican party’s president.

Now Mrs Foster is a political leader transformed. As the sun shone on Westminster this week, she arrived at 10 Downing Street with the fate of Theresa May and the UK government effectively in her hands. The DUP emerged as kingmakers after the June 8 general election failed to deliver the expected majority for Mrs May. As its 10 MPs were being courted by the Conservatives, Mrs Foster, who is not an MP, arrived in London ready to name her price.

The turnround in Mrs Foster’s fortunes, and the DUP’s, is partly due to the campaign mistakes of others, including Tory hubris and Sinn Féin triumphalism. Yet it is also a credit to her tenacity. A life shaped by Northern Ireland’s years of violence, known as the Troubles, a political career forged among the hard-faced men of Ulster unionism, and a penchant for rubbing Sinn Féin up the wrong way have turned this mother-of-three into what Mrs May herself might call a “bloody difficult woman”.

Ken Funston, who runs an advocacy group for victims of the Troubles near where she grew up, and knows her, says: “I can tell you that when Theresa May met Arlene Foster, the prime minister would have been in for a tough ride. She’s a strong woman, and I’ve no doubt she is aware the eyes of the world are looking at her.”

Says Ernie Wilson, another friend: “She hugs me every time we meet. I don’t think she gives too many of those.”

Mrs Foster is what is known in Ireland as a “border unionist”. Born Arlene Kelly in 1970 and raised in Roslea, a village near the frontier with the Republic of Ireland, she comes from a Protestant community of farmers and small-business owners whose lives have been defined over the past century by the presence of the border.

Twice during her childhood, the Troubles encroached on her family. When she was eight, the Irish Republican Army shot and injured her father John as he herded cattle. And as a 16-year-old, the bus on which she was travelling to the Collegiate Grammar School in Enniskillen one summer morning was blown up by an IRA bomb.

Mr Wilson was driving the bus that morning; he was the intended target. He still remembers the young schoolgirl’s preternatural calm in the moments after the explosion. “All the seats and windows had all been blown to pieces and I couldn’t see or hear a thing because I got hit in the head, but there she was, getting all the children out.”

That stoicism characterises the community of border unionists. They were targeted by the IRA relentlessly in a campaign of ethnic cleansing during the Troubles, with the aim of forcing the unionists to move away: the Fosters were rehoused.

Mrs Foster studied law at Queen’s University in Belfast, and later joined the Ulster Unionist party, then the dominant voice of her community. But she abandoned the UUP in 2004 for the DUP because she opposed the Good Friday Agreement — the accord that ended the Troubles and enabled power-sharing between unionists and republicans. Her defection was act of opportunism that made her political career.

The DUP was founded in 1971 by Ian Paisley, a bombastic Presbyterian preacher. He fused the ethnic chauvinism that is part of the DNA of Ulster unionism with the secular culture of “loyalists”, as working-class unionists are known. His brand of unionism acquired a militant edge which can still be seen on the party’s fringes today.

Paisley’s successor, Peter Robinson, made the DUP less beholden to its evangelical and anti-Catholic wing, and began to widen its appeal to urban professionals. Mrs Foster, a regular churchgoer, has not maintained the modernising momentum she inherited. Instead, she has cemented her leadership by “being more DUP than the DUP”, says Ben Lowry, deputy editor of the News Letter, a unionist newspaper in Belfast.

She has reconciled herself to the Good Friday Agreement and to working with Sinn Féin in Northern Ireland’s power-sharing institutions. Support for a minority Tory government is likely to depend on greater investment in Northern Ireland. But any political concessions to the DUP risk creating further tension with Sinn Féin if they are seen to undermine the Good Friday Agreement.

Northern Ireland is an often forgotten part of the UK, more socially conservative and divided than secular Britain. Mrs Foster, a royalist, reflects that gulf. Whether she can bridge it remains to be seen.

“I think she’s instinctively liberal on social issues like gay rights or abortion, but she doesn’t have the confidence to make the party more moderate,” Mr Lowry says. But having led the party to the gates of Downing Street, she may no longer need to.

The writer is the FT’s Ireland correspondent