Religion in the Republic of Ireland
The predominant religion in the Republic of Ireland is Christianity, with the largest church being the Roman Catholic Church. The Irish constitution says that the state may not endorse any particular religion and guarantees freedom of religion.
Catholic Church in Ireland
The Roman Empire never reached Ireland; so when the Edict of Milan in 313 AD allowed tolerance for the Levantine-originated religion of Christianity and then the Edict of Thessalonica in 380 AD enforced it as the state religion of the Empire; covering much of Europe (including Roman Britain); the indigenous Indo-European pagan traditions of the Gaels in Ireland remained normative. Aside from this independence, Gaelic Ireland was a highly decentralised tribal society, so mass conversion to a new system would prove a drawn out process.