As it turned out, Brexit had a profound impact on Ireland — not least that it put the constitutional question firmly on the table. 

Before the Brexit referendum, very few people expected a border poll on Irish unity to take place within the following 25 to 50 years. Northern Ireland was settling into peaceful coexistence. Once implacable foes, the DUP and Sinn Féin ruled together in a power-sharing administration. 

But everything changed the day after the result was called: Martin McGuinness, the Sinn Féin deputy first minister of Northern Ireland, called for a border poll. For the first time in living memory, the main political parties in Dublin began addressing the issue of Irish unity.

The genius of the Good Friday agreement is that it smoothed over grievances that had plunged Northern Ireland into decades of bloody conflict. 

People could choose to be British, Irish, Northern Irish, European, or all of these. What’s more, the border between the six counties of Northern Ireland with the 26-county southern state — a source of deep antagonism for many in the nationalist community — had been made invisible through the UK and Ireland’s common membership of the EU.

Brexit sundered the legislative underpinning of the historic 1998 peace accord. People were once again forced to choose between being British or Irish as the UK’s exit from the bloc undermined Northern Ireland’s “shared space” that enabled people to have fluid identities.

More importantly, however, Brexit tested Northern Ireland’s position within the union, perhaps more than any other event over the past century. 

Hardline Brexiteers wanted a complete exit from the EU, including leaving the customs union and single market. The EU insisted the UK honour its obligation to maintain a seamless border between the north and south of Ireland as part of the peace accord. The two positions were irreconcilable.

To break the impasse, the Irish government proposed putting the border down the Irish Sea, which prompted a furious backlash from unionists. They said they would never accept any border that put Northern Ireland on a different constitutional footing from the rest of the UK. 

They were given reassurance in 2019 when, during his campaign to become prime minister, Boris Johnson gave a solemn pledge to unionists that he would never accept a border down the Irish Sea. 

In October 2019, he signed a deal that put a customs border down the Irish Sea. Even though Johnson had no intention of honouring this legally binding agreement, it was a profound jolt to unionism.

Opinion polls show the majority of people in the Republic of Ireland want a border poll within five years and will vote for a united Ireland, with caveats. People in Northern Ireland want a border poll over a 10-year timeframe, but the majority want to stay in the union.

In reality, it is impossible to gauge the appetite for unification among people on both sides of the border until they know what a “new” Ireland would look like. 

Is it a reversal of the current arrangement whereby the devolved Stormont administration could continue in a federal Ireland, with unionists free to maintain their British citizenship? Would a united Ireland have a new flag? A new constitution? A new national anthem? 

These are extremely complex issues, not least for people in the republic, who cherish these symbols and are reluctant to give them up.

If there is to be a referendum, there needs to be extensive dialogue between nationalist and unionist communities in Northern Ireland and the south. 

The Irish government is wary about planning for unification in case it further destabilises an already fragile political backdrop in Northern Ireland. 

Meanwhile, the main unionist political parties refuse to engage in any talks about a united Ireland, although they privately acknowledge a border poll is coming. Just as republicans never accepted British rule in Northern Ireland, hardline unionists and loyalists will never accept a united Ireland.

Ultimately, a border poll will be decided by the growing middle ground in Northern Ireland — moderate unionists and nationalists and those who identify as neither. They will want reassurance over identity and rights in any new dispensation, but by far the biggest factor that will sway their votes is economics.

This is the biggest challenge facing advocates for a united Ireland. People in the republic will not vote for unification if it means paying more taxes. People in Northern Ireland will not vote to make themselves poorer. 

The British government transfers about £10bn every year to plug Northern Ireland’s fiscal deficit. Not only would Dublin have to guarantee this annual subvention, it would have to show Northern Ireland could prosper after unification.

Again, Brexit could play a role. Ireland’s economy is booming thanks in part to investment that would have otherwise gone to the UK before its departure from the EU. The UK economy has been suffering since the 2016 vote. 

Ironically, Brexiteers such as Farage have done more for Irish unity than the countless Irish republicans who have made blood sacrifices in the name of freedom over the past few centuries.

• John Walsh is a Dublin-based journalist, and writer and co-producer of the feature-length documentary The Irish Question, which premiered at Dublin international film festival

• The Guardian