The Second Prison by Ronan Bennett
Kane is part of a group of young Irish republican activists, whose belief in their cause at times competes with an urge to inform the police. After he is let out of jail for his alleged part in the murder of another informer, Kane goes to England to seek out and avenge himself on the final surviving member of his cell. Here he is drawn ever more deeply into the web of Tempest, the psychotic Special Branch policeman who destroyed Kane's cell in Belfast. Kane's encounter with him closes the circle of this taut, compelling novel.