Farmer John Sheridan drives his mud-caked Land Rover up and down country lanes and roads, back and forth across the snaking border separating Northern Ireland and the Republic of Ireland.

Britain and the EU agreed Friday that after Brexit there must be no return to a "hard border" between Northern Ireland, which is part of the U.K., and Ireland, which will remain a member of the EU.

Outside Northern Ireland, few people in Britain were thinking about the border when the country voted in June 2016 to leave the European Union. But the 500-kilometer line will be the United Kingdom's only land border with an EU country once Britain leaves the now 28-nation bloc in March 2019 .

There has only been an international border in Ireland since 1921, after a war that saw mostly Catholic Ireland break free of Britain – apart from six counties in the Protestant-majority north, which remained in the U.K.

The border began to blur after the EU's single market for goods, services and people was born in 1993, with both Britain and Ireland among its members.

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