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 the outskirts of Belfast on Saturday, she appeared confident and calm. She smiled as applause erupted from supporters in the balcony. Yet her determined walk and 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.