The Ghosts of Belfash by Stuart Nelville

Fegan has been a "hard man," an IRA killer in northern Ireland. Now that peace has come, he is being haunted day and night by twelve ghosts: a mother and infant, a schoolboy, a butcher, an RUC constable, and seven other of his innocent victims. In order to appease them, he's going to have to kill the men who gave him orders.