Excerpts from Books and Wikipedia
"But, in the eyes of those for whom there was only one correct Christianity, the Irish version had come to seem distinctly odd by the later eleventh century. . . . Pope Adrian IV authorized the invasion of Ireland by the English king Henry II (1154-89) on the grounds that it would be to Henry's eternal credit 'to enlarge the boundaries of the church, to reveal the truth of the Christian faith to peoples still untaught and barbarous, and to root out the weeds of vice from the Lord's field'. The Irish, then, were all but outright pagans, outside the church: their lack of correct Christianity justified invasion and conquest." [Europe after Rome, p. 238-9] "The Norman invasion in 1169 resulted again in a partial conquest of the island and marked the beginning of more than 800 years of English political and military involvement in Ireland. Initially successful, Norman gains were rolled back over succeeding centuries as a Gaelic resurgence reestablished Gaelic cultural preeminence over most of the country, apart from the walled towns and the area around Dublin known as The Pale." [Wikipedia]