They mean that it is simply ridiculous to pretend that Northern Ireland is in every respect the same as Britain. Those who oppose her backstop plan do not understand the UK union. But, from the context of her comments, she appears to regard the union simply as the union with Northern Ireland. Yet this is only one part of the UK union, and it is not a typical one. The union with Northern Ireland emerged from a treaty with Irish nationalists in the early 20th century.

Source: The Guardian December 25, 2018 12:00 UTC