Today Northern Ireland, comprising most of the ancient province of Ulster, sits simultaneously inside the European Union Single Market (largely under EU trading rules and standards) and also within the United Kingdom, which has left the European Union and its single market.

How can that possibly be? The answer lies partly in the terms reached before the recent U.K. departure from the EU and partly deep in the Irish question, which has bedeviled British politics for generations, and indeed for centuries. The nature of the Irish question may change in form over time, but the issue in all its tortuous complexity is always there — and is now back once again with a vengeance. Partition of Ireland was the solution in the last century, but now it persists as the impossible problem in this one.

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