One of the almost unsolvable problems with the U.K.’s exit from the E.U. is that it would necessitate a “hard border” between Northern Ireland, which is part of the U.K., and the Republic of Ireland, which would remain a member nation in Europe. The border was the epicenter of bloody conflict during the decades-long Troubles, and was essentially dismantled during the peace established by the Good Friday Agreement, in 1998. The prospect of fortifying it, with customs-and-immigration checks, has already brought threats of violence from paramilitaries such as the New I.R.A. At the same time, moving the customs border to ports along the coast of Northern Ireland—as the U.K.’s Prime Minister, Boris Johnson, has proposed—strikes Northern Irish loyalists as a step toward unification with the Republic, which they would view as an abandonment by Britain. Patrick Radden Keefe, who wrote about the Troubles in his book “Say Nothing,” discusses the intensely fraught issues of the border with Simon Carswell, the public-affairs editor of the Irish Times. Plus, the writer Carmen Maria Machado takes us back to a farmers’ market of her childhood.