BELTURBET, Ireland — When he crossed into Northern Ireland recently, Patrick O’Reilly felt his body tense up, a feeling that was once all-too familiar but that he thought would never return. Yet now, the fear and anxiety were creeping back, like a long-lost relative suddenly appearing out of nowhere, looking for trouble.

Mr. O’Reilly blames Britain for stirring up those grievances. Its expected departure — possibly a messy one — from the European Union, known as Brexit, is reopening old wounds and resentments in Ireland against its former colonial master.

“The British,” said Mr. O’Reilly, a retired pub manager, as he wove back and forth across the barely detectable border in his car, “are about to kick us in the teeth again.”

In the tortured history between the two island nations, Brexit is just the latest in a long line of perceived slights the Irish have suffered at the hands of the British. And now, with the possible exception of Britain, no country stands to lose more from Brexit, and particularly from a damaging “no-deal” departure, than Ireland.