Great Britain’s decision to leave the European Union has stirred talk of reuniting the island of Ireland as one political as well as geographic entity. It’s not going to happen soon (this year, next year…), but Brexit makes it more likely such an effort will be tried, whether successful or not, before the centennial of Irish partition in 2021. Here’s more background:

Why did Ireland split up, anyway?

How much time have you got? In the World War I era, Irish nationalists were close to obtaining limited domestic autonomy, called home rule, while remaining within the United Kingdom of Great Britain and Ireland, which was created in 1800. The effort split internally as more militant nationalists, or republicans, demanded full independence from Britain. The Protestant majority in the northeast province of Ireland, called Ulster, wanted to keep the status quo, hence the term unionists. The May 1921 partition of Ireland was an attempt to keep both sides happy. Six counties in the northeast were renamed Northern Ireland and remained part of Britain. The other 26 counties of the island, predominantly Catholic, were at first called the Irish Free State, then later became the fully independent Republic of Ireland. Read a more detailed history on “The Emergence of the Two Irelands.”

A 1937 map shows Irish Free State (south) and Northern Ireland.

What impact does Brexit have on this arrangement?

Voters in Northern Ireland voted by 56 percent to 44 percent to remain in the European Union (joining Scotland and the city of London in opposition to Brexit), but the overall referendum passed by 52 percent to 48 percent. Though leaving the E.U., Northern Ireland remains part of the U.K. Got that? Now, instead of a soft border between two E.U. countries (Ireland and U.K.), a hard divide will be created between E.U. and non-E.U. nations. It will be more difficult for people and goods to cross the border.

What about reuniting the ‘two Irelands’ so both are in the E.U.?

The 1998 Good Friday Agreement that created a power-sharing (home rule) government in Northern Ireland contains a provision for a “border poll” on becoming part of a united Ireland. The Irish nationalist Sinn Féin party immediately called for such a referendum after the Brexit results were announced. “Not so fast,” responded British Northern Ireland Secretary Theresa Villiers and unionist politicians. As The Guardian reports, “there cannot be a poll on Irish unity or remaining within the U.K. unless the majority of political representatives of both communities in Northern Ireland demand it.”

What does the polling say?

Last fall, an RTÉ/BBC cross border poll showed that just under one third of those surveyed in Northern Ireland favored political reunification of the island within their lifetime, compared to two thirds of respondents living in the Republic of Ireland. It’s important to remember that the poll was taken months before the Brexit vote. A sustained economic downturn resulting from Brexit may prompt Northern Ireland to embrace the Republic. Historical note: A 1973 referendum in Northern Ireland asked whether people wanted to remain in the U.K. or rejoin Ireland. The remain vote won by a landslide 98 percent, but Catholic nationalists boycotted the election for a variety of reasons. Of course, 1973 was just beginning of The Troubles, and long before economic globalization.

What else could happen in Northern Ireland?

There are already suggestions that Northern Ireland might join Scotland, if and when it splits from Great Britain as the result of Brexit. Northern Ireland and Scotland have shared historic and cultural ties. It’s also possible that a few of the six counties in Northern Ireland could rejoin Ireland, especially those on the border, while the others remain linked to Britain. Or Northern Ireland could opt for its own independence.