Originally published in 1966, traces the history of the church right up until the twelfth century. It gives an account of the problems which arose when the organization of the Christian church, imported from the urban bureaucracy of the Roman Empire, had to be adapted to the society of early Ireland. The book also looks at the legal texts of the sixth seventh and eighth centuries and attempts through them, to trace the gradual process of modification which culminated in the eighth century, when the church now fully adjusted to Irish society, reached a so-far unprecedented height of power and influence. The book also examines the issues faced in the ninth century by the Viking raids and settlements.