Ireland's Holy Wars - the Struggle for a Nation's Soul 1500-2000 by Marcus Tanner (a former foreign editor of the Independent in London) is the newest tome on the age-old problem. The press release from Yale University Press states that during 18 months of research the author "researched archives at the National Library in Dublin, walked with Apprentice Boys on their controversial annual marches, sat drinking with unionists in fiercely sectarian bars and talked with Catholic widows in the Bogside of Belfast". So plenty of knowledge there then.