When Christianity came to ancient Ireland, what happened to the druids? They can't just have been wiped out, because there was no centralized state power structure that could do such a thing, and in any case the Irish conversion was largely peaceful.

In Celtic Gaul, there were supposed to have been three intellectual classes: the druids, the bards and the oracles or prophets. “Bard,” in medieval Wales, meant a high-status professional poet. But in medieval Ireland, a high-ranked poet was called a “fili,” and the term “bard” was reserved for a common minstrel. Why the difference in terminology?

Many Celtic scholars now believe that the fili and the druids were the exact same class, and that those druids who were willing to accommodate themselves to the new Christian religious landscape simply changed what they called themselves and continued on in much the same role as before. Fili were seen as having many strange powers, and the medieval Irish church was always trying to get them to drop their more overtly pagan practices. For instance, fili were supposed to practice a ritual where they made offerings to spirits- in other words, a pagan religious sacrifice- and then went to sleep to seek a prophetic dream. This was one of the “druidic” practices banned by the church, presumably only once it was well-established enough to be able to challenge the lingering remnants of druidism among the elite poets.

The fili, like the druids before them, had tremendous power and influence in Gaelic society, a status they only lost when the Gaelic warrior aristocracy was finally destroyed by the English. The Hebridean writer Martin Martin of Skye, writing in the early eighteenth century, goes so far as to claim that Highland chiefs received their inauguration at the hands of the local “druid” even at that late date. Scholars have tended to gloss over his use of that word, since there can't possibly have been any druids in the strict sense in the eighteenth-century Hebrides. In all likelihood, Martin is referring to a fili, but it is possible that the term “druid” was sometimes being used for the poets at that time, which would strongly support the idea that they were originally the same thing.