The Brexit soap opera continues, with all eyes now fixed on the Irish frontier. The land border between Northern Ireland and the Irish Republic is open, but it will be difficult to keep it that way once the U.K. leaves the European Union. Late last year Theresa May’s government accepted Brussels’ demand for an “Irish backstop” in its Brexit plan—a commitment to introduce a customs border between Northern Ireland and the rest of the U.K. so that cross-border travel and trade would remain seamless. The unpopularity of this idea among U.K. Brexiteers is the chief reason Parliament voted down Prime Minister May’s proposed...