The writer is an FT contributing editor

The force lay in the symbolism. The parliament buildings at Stormont stood as the citadel of Protestant unionism. This month’s elections have left republican Sinn Féin holding sway in Northern Ireland’s assembly. The discomfort reaches well beyond unionism. The rise of the party that served as the political voice of the IRA also challenges the old order in the south.

When the Dublin government surrendered its territorial claim to Northern Ireland in the 1998 Good Friday Agreement, the aim was to offer reassurance to those whose allegiance was to the UK. Unification of the island of Ireland would happen only with the consent of the (then unionist) majority in the six counties in the north. As a quid pro quo, the accord provided for power sharing at Stormont, an open north-south border and a voice for Dublin in the affairs of the province.

As much as this rewriting of Eamon de Valera’s 1937 constitution was presented as a concession to unionism, it also happened to fit the temper of the times in the south. The Republic had turned its back on the closed, conservative and stiflingly Catholic politics of its most revered political leader. No one would say it out loud, but the status quo suited Dublin.

These were boom times for the “Celtic tiger”. Ireland’s economic growth was accompanied by social emancipation, which loosened the grip of the church. The Republic’s new place as a modern European state set it free from centuries of self-definition as anti-British.

The contrast with the north, falling behind economically and scarred by decades of sectarian violence, was stark. As much as unity was an article of faith, visitors to Dublin were assured at the highest levels that no one was in any great hurry.

Ambition sat comfortably on the far horizon. To adapt St Augustine, “Please make us whole, only not quite yet”. Unification would flow naturally through economic integration and political osmosis. Business would cement a new relationship. The eventual disappearance of the border would be scarcely noticed.

Time, however, now seems to be in an inconvenient hurry. Sinn Féin’s success at Stormont has been matched by its newfound strength in the Republic. For the century since partition, power in the Republic has belonged to Fianna Fáil and Fine Gael. At present they govern in coalition. But Sinn Féin’s brand of leftist populism has given it a strong lead in the opinion polls. For a new generation squeezed by the rising cost of living and an acute housing shortage, the days when Sinn Féin represented the ballot box placed alongside the IRA’s Armalite are ancient history.

To be clear, none of this is to say that Irish unity is suddenly around the corner. Sinn Féin’s success at Stormont owed more to a splintering of unionism caused by Boris Johnson’s hard Brexit than to a shift in the popular mood towards unity. Likewise, demography’s steady erosion of the Protestant majority in the six counties has yet to be translated into an upsurge in nationalism. The other big winner in the elections was the nonsectarian Alliance party. For now, plenty of Catholics seem content with the status quo.

The Good Friday Agreement ended most of the violence, but it did not heal the wounds. The Democratic Unionist party’s present veto on the creation of a new Stormont executive in protest at the province’s post-Brexit trading arrangements with the EU speaks to an abiding fear of betrayal by governments in Westminster. Johnson promises to tear up the so-called Northern Ireland protocol, but for the prime minister this is a matter of political opportunism, calculated to appease his party’s Brexit fundamentalists, rather than a measure of commitment to the union.

For all these complications and caveats, the political dynamic has changed irrevocably. The Dublin political establishment cannot any longer treat a united Ireland as a distant aspiration. Sinn Féin’s advance puts an unanswered question back at the heart of Irish politics: what would a united Ireland look like?

No one seems to know. Voters in the Republic say they would back unification by a sizeable margin. But the same opinion polls also say such support is predicated on the notion that nothing much would change. The border would disappear but life would continue as before.

Except that it couldn’t. Beyond the costs (Northern Ireland relies on an annual subsidy of £12bn from the UK), the new state would include a million people identifying as British. The unity imagined by hardline nationalists — a narrow referendum win in the North celebrated as a victory for nationalism by the South — would invite the return in a united Ireland of the resentments and sectarianism that so disfigured the north. To work, the new state would need a new constitution. Perhaps a federal structure with a new flag and anthem to recognise the legitimacy of unionism?

There are doubtless answers to all these questions. But finding them will require mutual respect and cross-community consensus. This is not a process to be hurried to fit the impulsive demands of everyday politics. A century ago the nationalist cause in Ireland could be simply described: to end British rule. Now it must find a welcome for unionists.