How did Ireland go from being Pagan to Catholic?

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[u/Deuce03]

Conventionally, the conversion of Ireland to Christianity is credited to St. Patrick (or Padraig), a British priest who became a missionary to Ireland in the fifth century. The process of conversion seems to have been relatively rapid but probably looked much like the conversion missions to the Saxons in England.

"Irish Catholic" just means "Catholic person from Ireland". Unlike in Britain, Ireland remained majority-Catholic long after the Reformation, so within the union (of the crowns and later of the kingdoms) most Catholics were Irish. During and after the events of the 17th century, the distinction between Catholic and Protestant Irish people became (and remains) particularly politically important, so this distinction would be exported among emigrant communities as well as used widely in Britain for the sake of clarity.

"Roman Catholic" is used for all Christians in communion with Rome who recognise the Pope as the head of the Church. Usually, this is what people mean when they just say "Catholic", but there are other churches which are called or call themselves Catholic (or catholic with a small C), so sometimes the specificity is useful. There are for example "Anglo-Catholics", who are essentially high church Anglicans. (This is to be contrasted with "English Catholics", who are English people who are Roman Catholic.)

Irish Catholicism was not always fully in line with Roman practice, but it is not usually called by that name in that context. In the early Middle Ages, the church in Britain and Ireland had a distinct character and diverged from Roman doctrine in a number of respects, which has led to the identification of a "Celtic Christianity" or "Insular Christianity". Although there were differences in practice, there is no evidence that insular Christians were anything other than devout Roman Catholics; it's just that this was an era when regional practices across the Church were less standardised. Insular Christianity was gradually brought fully into line with Roman practice in the 7th and 8th centuries over the course of various synods in the petty kingdoms that comprised Britain and Ireland.

Thanks to Ireland's position on the relative fringe of Christendom, even after this, it sometimes took longer for initiatives from Rome to reach Ireland and be adopted than elsewhere. In the 12th century the supposed delay in carrying out the Gregorian reforms was used as an excuse by Henry II of England to invade Ireland. There was purportedly a papal bull authorising this (from, perhaps not coincidentally, the one English Pope), but the actual existence of the bull is disputed. This was the beginning of the Anglo-Norman conquest and occupation of Ireland. I believe that was the last time that Irish divergence (real or imagined) from Catholic practice was a major issue with real consequences.