The most intolerable strain would lie across the Irish Sea. A Brexit that doesn’t include an Irish backstop could stoke old tensions, explained my colleague Siobhn O’Grady: “The border is a highly sensitive issue in Ireland because its current arrangement was negotiated as part of the 1998 Good Friday Agreement that put an end to the 30 year period known as ‘the Troubles.’ During that time, sectarian violence between Unionists, who wanted Northern Ireland to stay part of the United…