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Northern Ireland's Sinn Fein politicians Mary Lou McDonald and Michelle O'Neill talk to the media outside the Houses of Parliament, as uncertainty over Brexit continues, in London, Britain, April 8, 2019.

GONZALO FUENTES/Reuters

The deepening uncertainty surrounding Brexit is fuelling calls for a referendum on the future of Northern Ireland and whether it’s time for the province to rejoin Ireland.

“I strongly believe that the genie is out of the bottle,” Mary Lou McDonald said on Monday during a press conference with foreign journalists. Ms. McDonald is the president of Sinn Fein, which has led the charge for Irish reunification for decades and opposes Brexit. “Brexit is a constitutional earthquake and it puts the issue of the partition of Ireland front and centre.”

Support for Irish reunification has been growing ever since the 2016 referendum on Britain’s membership in the European Union. While Britain as a whole voted 52 per cent to leave the EU, 56 per cent of people in Northern Ireland voted to remain. Sinn Fein campaigned for the Remain side while its rivals in Northern Ireland, the Democratic Unionist Party (DUP), backed leaving the EU.

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Concern about Brexit has intensified in Northern Ireland now that Prime Minister Theresa May has failed to win approval in Parliament for a withdrawal agreement. The main sticking point has been the fate of the Irish border and whether it will remain control-free. Many economists and business leaders in Northern Ireland worry that if Britain crashes out of the EU without a deal, that will lead to a hard border with Ireland, which could cripple the economy. It would also threaten the peace under the Good Friday Agreement. Ms. May has launched last-ditch talks to find a way out of the impasse, but there are only days to go before Britain is slated to leave the EU on Friday. On Wednesday, Ms. May will ask EU leaders to extend the deadline until the end of June.

A November poll conducted in Northern Ireland for the BBC and Ireland’s RTE broadcaster found that 62 per cent of those surveyed believed Brexit made a united Ireland a more likely possibility. Other polls have shown support for a reunification approaching 50 per cent in Northern Ireland and Ireland, especially if Britain leaves without an agreement.

Sinn Fein has seized on the discord to push for a referendum on unification. “Even with [a Brexit deal], we are faced with a major, major challenge and a fundamental shift in the relationship between our island and Britain,” Ms. McDonald said. “All of those things lead you to the one obvious conclusion that you actually have to say out loud; let’s talk about this.”

The British government divided Ireland in 1921 as a way of ending the continuing struggle for Irish independence. Six predominantly Protestant counties were combined into Northern Ireland while the remaining 26 mainly Catholic counties became Southern Ireland. The south became the Republic of Ireland in 1949 while Northern Ireland remained part of Britain.

Sinn Fein, which operates in Northern Ireland and Ireland, has been at the heart of the battle for a unified country for more than 100 years. It has often been referred to as the political arm of the Irish Republican Army, although Ms. McDonald, an MP in the Irish Parliament, has played down the IRA connection since taking over as party president last year from Gerry Adams.

Ms. McDonald said the party has called on the Irish government to set up a public forum to begin discussions about a united Ireland, and she has demanded that Ms. May hold a referendum on Irish unity. “This is now very much a conversation that is under way,” she said.

Holding a referendum isn’t easy. Under the terms of the 1998 Good Friday Agreement, which ended decades of sectarian violence, Britain’s Secretary of State for Northern Ireland must call for a vote if it appears likely that a majority of people in Northern Ireland would support Irish unity. A separate vote would also have to be held in Ireland. So far, the British government has ruled out holding a referendum and while polls show increasing support for unification, it’s unclear there’s a majority on both sides of the border.

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Ms. May’s minority government is also propped up by the DUP’s 10 Members of Parliament who fiercely oppose reunification. DUP Leader Arlene Foster has lashed out at Sinn Fein for engaging what she called “project fear” and she said the conditions for a referendum have not been met.

Sinn Fein members have also come under criticism for failing to take their seats at Westminster. The party elected seven MPs in Northern Ireland in 2017, but Sinn Fein has a policy of abstaining from Westminster. Many of those who support a soft Brexit and keeping close ties to the EU have criticized Sinn Fein for staying away from Parliament because several key Brexit motions have been lost by a handful of votes. Ms. McDonald stood by the party’s policy, insisting that Sinn Fein MPs would never swear allegiance to a foreign power. “Westminster is by design and intent there to pursue British interests. So we have no business in there,” she said.