The Flight of the Earls (the O'Neils, the most influential clan in Ireland) in 1607 to Spain left a sort of power vacuum that was filled with the Plantation of Ulster (Ulster is what we call Northern Ireland today), which brought (mainly) protestants over from Scotland and England to spread "civilized" culture and Protestantism.

You can read about this in a number of books, but The Course of Irish History is a good survey.