When Saint Patrick began his missionary work in Ireland in the early fifth century, he encountered there a people who had never before had any knowledge of God but had until then only worshipped ‘idols and unclean things’. This statement comes from his Confessio, the famous autobiographical document in which he defends the divine justification of his mission and provides a retrospective reflection on his own spiritual growth. As envisaged by Patrick, the conversion of the Irish marked the ultimate fulfilment of the apostolic command of Christ: by leading them away from idolatry and making them a ‘people of the Lord’ (cf. 1 Thessalonians 1:9), he had completed the final stage in a process that prepared the way for the Second Coming of Christ in the Last Days.

We know today that Christianity had already reached parts of Ireland prior to Patrick's mission, and that the process of Christianization was a lot longer and more complex than what could be inferred from Patrick's words. But what more can be said about these ‘idols and unclean things’ that the Irish in their ignorance were said to have worshipped as their gods?

The nature of Ireland's indigenous belief system has been the subject of extensive scholarly speculation, in which evidence adduced from the archaeological and literary record has been utilized to reconstruct different aspects of ‘pre-Christian’ or ‘Celtic’ religion.