People are talking up the prospects of a united Ireland. It’s easier said than done.

Still, the possibility of Irish reunification will hang over many of the political discussions this St. Patrick’s Day. Brexit has reignited tensions and fueled interest in a referendum. In the 2016 Brexit referendum, most of Northern Ireland voted to remain in the European Union. Unification would accomplish that. The U.K.’s Brexit negotiations with the European Union on where to conduct customs checks on goods moving between Northern Ireland and the Republic was also resolved in a way that shifted the political gravity from London to Dublin. Indeed, the final Withdrawal Agreement included a protocol that created an administrative border in the Irish Sea rather than reimposing a hard land border between the Republic and Northern Ireland. Finally, some public opinion polls suggest that Irish unity is becoming more popular in Northern Ireland.

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So, what would a referendum on a United Ireland involve? Here’s what you need to know.

A unification referendum would require a lot of advance work

The Belfast or Good Friday Agreement of 1998 ended the violent phase of recent conflict in Northern Ireland. All the groups involved in the conflict committed themselves to accepting that Northern Ireland would remain part of the U.K. as long as a majority supported it. However, the Agreement obliges the British government to initiate a referendum on unification whenever it appears a majority would support it.

That sounds clear enough — but a lot of difficult questions would have to be answered before a referendum happened. That’s what the interim report of the Working Group on Unification Referendums on the Island of Ireland at University College London makes clear. Most obviously, what exactly would people vote on? Unification could take many forms. A new Ireland could be federal or unitary. It might or might not retain the power-sharing institutions created by the Belfast Agreement, shaping who has power in the new government. The Working Group argues that, before any referendum, the Irish government needs to hold inclusive discussions with the British government, politicians from Northern Ireland, civil society leaders and the general public to decide either the form of the new Ireland or a process for working that out.

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That’s not the only question. The British health system is the single most important issue driving opposition to Irish unification in Northern Ireland. Citizens of Northern Ireland get free medical care as part of the U.K.’s socialized medicine. British taxpayers subsidize Northern Ireland’s public services to the tune of $12.5 billion, according to one estimate. That’s a lot of money to give up for the nationalist cause. Nor is it clear more generally what the costs of unification would be or who would pay them, or what the economic benefits of unification might involve and who would get them.

And of course, political questions are key. How would Unionists, who want Northern Ireland to remain in the U.K., be persuaded that a united Ireland would protect their interests? If a majority voted for a united Ireland but a large minority opposed that result, a referendum could result in an unworkable new government where all sides lack trust and incentive to cooperate.

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Running the referendum would be complicated

Even if these initial questions get sorted out, actually running the referendum would be complicated. As the Working Group report makes clear, changing the Irish constitution and integrating relevant parts of the Belfast Agreement would require a referendum not just in Northern Ireland, but also in the Republic of Ireland. Which should vote first, and how close together should they be? Northern Ireland and the Republic have different voting rules. Should European Union citizens be allowed to vote in both jurisdictions? How should the governments handle campaign contributions, especially contributions from abroad? And how should they minimize misinformation, which was a big problem in the Brexit referendum?

However these questions are answered, they probably would destabilize Northern Ireland’s institutions, created by the Good Friday agreement. Thanks to these institutions, Nationalists and Unionists share power. A referendum would be sure to divide them, disrupting a government that is already fragile. If the referendum fails, the agreement states that there cannot be another for seven years. At that point, it would be hard to see how politicians could work together.

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The Republic’s government is reportedly interested in Irish unification — but only over the very long term, to reduce the risk of disruption. Even Sinn Fein, which has a hold in both the north and the Republic, might prefer to delay a referendum until it has built more power in the Republic. Currently, a high proportion of young voters there support Sinn Fein; a difficult referendum could result in unpopularity and chaos.

In other words, calling for an immediate referendum on unification may be good politics for Sinn Fein and its supporters — but the groundwork hasn’t been laid.

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Carolyn Gallaher is a professor and senior associate dean in the School of International Service at American University, an author of “After the Peace: Loyalist Paramilitaries in Post-Accord Northern Ireland (Cornell University Press, 2007).