For the past two decades, you could cross the border between Northern Ireland and the Republic half a dozen times without noticing or, indeed, without turning off the road you were travelling. It cuts through fields, winds back-and-forth across roads, and wends from the mouth of the Newry River to the mouth of the Foyle. It's frictionless - a feat sealed by the Good Friday Agreement. Before that, watchtowers loomed over border communities, military checkpoints dotted the roads, and bridges had been demolished to prevent crossings. This is a past that most are happy to have left behind but it may also be the future. Diarmaid Ferriter charts its history from the divisive 1920s Act to the Treaty and its aftermath, through 'the Troubles' and the Good Friday Agreement up to the Brexit negotiations.