Northern Ireland Has a Sinn Fein Leader. It’s a Landmark Moment.

As Michelle O’Neill walked down the marble staircase in Northern Ireland’s Parliament building on Saturday, she appeared confident and calm. She smiled briefly as applause erupted from supporters, but her otherwise serious gaze conveyed the gravity of the moment.

The political party she represents, Sinn Fein, was shaped by the decades-long, bloody struggle of Irish nationalists in the territory who dreamed of reuniting with the Republic of Ireland and undoing the 1921 partition that has kept Northern Ireland under British rule.

Now, for the first time, a Sinn Fein politician holds Northern Ireland’s top political office, a landmark moment for the party and for the broader region as a power-sharing government is restored. The first minister role had previously always been held by a unionist politician committed to remaining part of the United Kingdom.

“As first minister, I am wholeheartedly committed to continuing the work of reconciliation between all our people,” Ms. O’Neill said, noting that her parents and grandparents would never have imagined that such a day would come. “I would never ask anyone to move on, but what I can ask is for us to move forward.”

The idea of a nationalist first minister in Northern Ireland, let alone one from Sinn Fein, a party with historic ties to the Irish Republican Army, was indeed once unthinkable.

But the story of Sinn Fein’s transformation — from a fringe party that was once the I.R.A.’s political wing, to a political force that won the most seats in Northern Ireland’s 2022 elections — is also the story of a changing political landscape and the results of the 1998 Good Friday Agreement, which ended the decades-long sectarian conflict known as the Troubles.

“It’s certainly symbolically very significant,” said Katy Hayward, a professor of political sociology at Queen’s University, Belfast. “It tells us just quite how far Northern Ireland has come, and in many ways the success of the Good Friday agreement and use of democratic and peaceful means of achieving cooperation.”

It is not yet clear what a Sinn Fein first minister will mean for the hopes of those who want to reunite the island after a century of separation. Although Mary Lou McDonald, the president of Sinn Fein, who leads the opposition in the Republic of Ireland’s Parliament, said this past week that the prospect of a united Ireland was now in “touching distance,” experts believe it remains far off.

For now, the territory’s two main political powers — unionists and nationalists — are locked together in the power-sharing arrangement that was laid out in the Good Friday Agreement.

That arrangement had collapsed over the question of how the political powers of Northern Ireland see themselves after Brexit.

Northern Ireland’s leading unionist party, the Democratic Unionists, quit the government in 2022, in the wake of Britain’s exit from the European Union, which had placed a trading border between Northern Ireland and the rest of the United Kingdom. Wanting to safeguard ties to Britain, the D.U.P. feared that the sea border was the first step to tearing them apart.

Its boycott of the assembly ended this past week after the British government agreed to reduce customs checks, strengthen Northern Ireland’s place within the United Kingdom and hand over 3.3 billion pounds, about $4 billion, in financial sweeteners.

Because it had the most unionist seats in the 2022 elections, the D.U.P. had the right to nominate the deputy first minister on Saturday — Emma Little-Pengelly, who will work alongside Ms. O’Neill.

“The past with all of its horrors can never be forgotten,” Ms. Little-Pengelly said as she described being a child during the Troubles and seeing the devastation of an I.R.A. bomb outside her house when she was 11. But she added, “While we are shaped by the past, we are not defined by it.”

The first and deputy first minister roles are officially equal, with neither able to act alone, to prevent either community from dominating the other. “People like to say here, one can’t order paper clips without the approval of the other,” Ms. Hayward said. But the titles, and the fact that the first minister’s role reflects the largest number of seats, creates a “first among equals” notion.

And Ms. O’Neill’s appointment has inevitably brought to the fore conversations about the prospect of Northern Ireland one day reuniting with the Republic of Ireland.

Experts said that while an ascendant Sinn Fein could provide further momentum to that cause, the party’s rise was more a reflection of the fractures that appeared among unionist parties after Britain left the European Union, rather than a widespread surge in Irish nationalism. Current polling suggests that the majority of the population across the island does not support unification.

“They’ve made the prospect look realistic, and Brexit helped, because support has increased somewhat,” said Jonathan Tonge, a professor of politics at the University of Liverpool who specializes in Northern Ireland, and who has extensively analyzed polling on the issue.

“It’s still got a distance to run,” he said, adding that with an election looming in the Republic of Ireland in 2025, and the potential for a Sinn Fein government there, “it’s huge in those terms.”

He noted that a quarter of a century ago, few would have envisaged a Sinn Fein first minister.

Part of that success is down to Ms. O’Neill and Ms. McDonald, who have helped change perceptions of the party.

“These two women don’t have the baggage of the membership or close association with the I.R.A.,” said Robert Savage, a professor at Boston College who is an expert in Irish history. “They are younger, articulate, popular and astute at addressing the concerns, particularly of younger people.”

Ms. O’Neill, 47, was born in Cork, a county on Ireland’s southern coast, into a prominent republican family from Northern Ireland. Her father, who served time in prison for being an I.R.A. member, later became a Sinn Fein politician. But she has already made an effort to frame herself as a first minister for all. She attended both Queen Elizabeth II’s funeral and the coronation of King Charles III last year.

Many unionists associate Sinn Fein with its I.R.A. history, as do some nationalists and those who do not identify with either group. But increasingly, particularly among a younger cohort, the party has proved appealing.

In the Republic of Ireland, the party won the popular vote in 2020, partly by focusing attention on social issues like housing and positioning itself as an alternative to the status quo. But its popularity did not extend to older voters who remember the violence of the Troubles.

In some ways, the growth of nationalist political representation is unsurprising. Demographics have shifted significantly in Northern Ireland, with the Protestant majority’s slow erosion there first attributed to the Catholic Church’s opposition to birth control and then to economic factors like the decline in industrial jobs, which were held predominantly by Protestants.

Catholics outnumbered Protestants in Northern Ireland for the first time in 2022, according to census figures. And Northern Ireland is not the binary society it once was. Decades of peace drew newcomers in, and like much of the world, the island has grown increasingly secular. The labels of Catholic and Protestant have been left as a clumsy shorthand for the cultural and political divide.

Since Brexit, there has been a fall in support for Northern Ireland’s remaining in the United Kingdom and a rise in support for Irish unification. Many voters saw the break from Europe as economically damaging and threatening to cross-border relations, as the island had enjoyed decades where E.U. membership helped shore up peace.

For now, the restored government in Belfast has more urgent issues to address. Last month, tens of thousands of public sector workers walked out in protest over pay, in Northern Ireland’s largest strike in recent memory. The health care sector is in crisis, and the rising cost of living has been felt more acutely there than anywhere else in the United Kingdom.

“Look at what happened when people did get around a table and work to create peace here, and the Good Friday agreement came from that,” said Paul Doherty, a city councilor who represents West Belfast, one of Northern Ireland’s most deprived communities. “I think we need to rekindle that spirit we had back in the ’90s.”