At a roadside diner in this small village in the far northwest of the Republic of Ireland, Kieran Harrigan contemplates a border that once loomed large but now seems barely to exist.

“The only way you know you’ve crossed the border is the color of the road markings,” says Mr. Harrigan, a retired construction manager, who has watched the frontier fade from identity flashpoint to negligible line.

It has been more than a century since the British government broke off six of Ireland’s 32 counties into Northern Ireland within the United Kingdom, with the remaining counties eventually becoming the independent Irish Republic. Partition sparked sectarian strife in Northern Ireland, which was resolved by the 1998 Good Friday Agreement, a multinational peace treaty that also set terms for how Nort

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