A couple of months after Roy Keane had left Saipan in 2002, Manchester United came to Dublin to play a pre-season friendly at Tolka Park. Outside the ground, you could buy T-shirts with pictures of Keane and Michael Collins with the slogan ‘Two Famous Cork Leaders Shot in the Back’. At the time, it seemed to capture the national mood. “There has never been an Irish sportsman or woman of this wattage,” Dave Hannigan writes in his new book, We Need to Talk About Roy, which is a brilliant study on Keane and what he has meant to Ireland, and…