This introductory chapter provides an overview of the history of Christianity in Ireland. Since the arrival of Palladius in Ireland in the year 431, the ‘land of saints and scholars’ has had a long and complex relationship with its dominant religion. Ireland's social, political and intellectual history has always been integrated with its experience of Christianity, for the religion that began to organize in 431 has shaped in powerful ways the island's cultures and languages, as well as its people's conceptions of what it means to be an individual, a family, a community, and a nation. Over one-and-a-half millennia, the Christian faith fashioned ideas about Irish-ness and Ireland, just as Irish missionaries, theologians, ideas, and experiences shaped Christianity as it expanded around the world. Nevertheless, the dominance of Christianity in Ireland was never complete and has never been uncontested.