BELFAST, Northern Ireland — It was meant to be a year of celebration.

But Northern Ireland, created in 1921 when Britain carved six counties out of Ireland’s northeast, is not enjoying its centenary. Its most ardent upholders, the unionists who believe that the place they call “our wee country” is and must forever remain an intrinsic part of the United Kingdom, are in utter disarray. Their largest party has ousted two leaders within a matter of weeks, while an angry minority has taken to the streets waving flags and threatening violence. And the British government, in resolving Brexit, placed a new border in the Irish Sea.

It’s harsh reward for what Northern Ireland’s first prime minister, James Craig, called “the most loyal part of Great Britain.” But the Protestant statelet is not what it was. Well on its way to having a Catholic majority, the country’s once dominant political force — unionism — now finds itself out of step with the community that traditionally gave it uncritical support. And for all his talk of the territorial integrity of the United Kingdom, Prime Minister Boris Johnson has made clear his government would cheerfully ditch this last little fragment of Britain’s empire if it continues to complicate Brexit.

The writing is on the wall. While the process by which Ireland could become unified is complicated and fraught, one thing seems certain: There isn’t going to be a second centenary for Northern Ireland. It might not even last another decade.

A hundred years ago, the mood among unionists was jubilant. When the king and queen of England came to Belfast to mark the opening of the new Northern Ireland Parliament, the streets were decked out with red, white and blue bunting. “The people could not contain themselves,” according to Cecil Craig, the wife of the new prime minister. “All Irishmen,” King George V said, should “join in making for the land which they love a new era of peace, contentment and good will.”

The Catholic minority, known as nationalists because they aspired to be reunited with the rest of Ireland, had no such expectations. For 50 years, unionism dominated the state, instituting a comprehensive system of discrimination in housing, education, employment and voting. Sectarianism was state policy — Protestants were instructed by their leaders to distrust and exclude Catholics, who were outnumbered two to one — and the police force was armed. Britain turned a blind eye, as did the Republic of Ireland.

But discontent among nationalists inevitably built, finding form in the late 1960s in a civil rights campaign that aimed to secure basic rights for the Catholic minority. Outraged, the unionist state reacted by attempting to beat peaceful protesters off the streets. The British Army, whose intervention quickly showed itself to be on the side of unionism, was confronted by the Irish Republican Army, which responded with its own brutal and sectarian campaign. In 1972 the British government suspended the regime in Belfast and placed Northern Ireland under its direct rule.

For almost three decades, the conflict raged. Around 4,000 people, out of a population of fewer than 2 million, were killed; communities were torn apart. In 1998, the Good Friday Agreement brought an end to the violence and inaugurated a power-sharing executive, in which parties representing the two main communities operate in mandatory coalition. It was ratified by 70 percent of people in a referendum. The war was over.

The arrangement stumbled along for close to two decades, never fully working yet crucially keeping the peace. But Britain’s vote in 2016 to leave the European Union threatened the state’s always fragile constitutional relationships. And when the Conservative government settled Brexit with a protocol that established a border for goods between Northern Ireland and the rest of the United Kingdom, it effectively acknowledged the province as a place apart.

Northern Ireland now has borders with Britain and Ireland — and it is no longer a majority-Protestant state. The last census, in 2011, showed that the Protestant population had declined to 48 percent and the Catholic minority had risen to 45 percent. The Protestant community is aging, too: In 2011, only among those over 60 did it have a significant majority, and among schoolchildren, Catholics were the larger group. The results of a census to be published next year may well show an overall Catholic majority.

Nor can unionists count on the votes of Protestants. As a society, Northern Ireland has become more secular, more tolerant of diversity, less insular. People who reject conservative social policies have other voting options, and many young people do not vote at all. Some put their energy into global movements like climate justice and feminism — and plenty neither know nor care about the religious background of their friends. The constitutional issue of whether Northern Ireland is Irish or British does not preoccupy them. They are open to persuasion.

Unable to adapt, unionism is on the wane. According to a recent poll, support for the Democratic Unionist Party has slumped to 16 percent, with Sinn Fein, the party that emerged from the I.R.A. and whose fundamental aim is to achieve a united Ireland, well ahead at 25 percent. The next elections, due in less than a year, could see Sinn Fein take the post of first minister for the first time, in what would be a symbolically momentous development.

What’s more, Sinn Fein is surging ahead in polls in the Irish Republic and may enter government after the next elections in 2025. While around 50 percent of Northern Irish voters back remaining in the United Kingdom, support for Irish unity is growing. Though by no means imminent, that goal has never seemed closer.

Against this backdrop, some unionists have sunk into resentment. Men in balaclavas, Union Jacks in their fists, have taken to the streets to express their grievances. But it’s clear that most Protestants, like the rest of Northern Ireland’s populace, deplore talk of a return to violence. They want normal politics instead.

And if unionism cannot deliver it, a growing number of them are tentatively contemplating what for previous generations was unthinkable: that a unified Ireland might not actually be the end of the world.