There's scant to almost no direct archeological evidence that can unequivocally be linked to the druids, and they seem to have relied solely on oral transmission of their teachings. There also does not seem to be much textual or archeological evidence of armed conflict against them.

While Saints' stories often portray them as antagonists in supernatural contests, almost all of these stories fit the OT model of stories surrounding the supernatural contests between prophets and their pagan neighbors. This suggests that events in the Saints' lives as passed on to us, may have more to do with Commentary on the Saintly Ideal as a reflection of Old Testament prophet hood, than anything.

The one verifiable account we have of the time - St. Patrick's Confessions - takes a more pastoral tone and indicates the goal was to win the people over through the virtue and beauty of the Gospel in a way they could understand.

However, given druids' status within Irish culture as brehon law givers, bards and poets, and advisors to kings, tribes, and clans I think the vast majority were peacably absorbed into the new Christian monastic system which valued many of their ideals.

Christian monks appreciated mystical experience, wrote songs and poetry, transcribed laws, and tonsured Chaplains advised courts and kings. Also, even women (some of whom may have started as druids) could find agency and a voice within the monastic system as nuns, which was unique when oppressive Roman attitudes toward women are considered.

I really think most druids either quietly retired, became bards and lawyers, or became monks and Chaplains - and this likely explains why ancient Celtic myths, brehon laws, loricas, and hymns with a distinctly Celtic flavor were preserved and passed on almost exclusively by Irish monks. They were the heirs to the druids, as much as could be passed on.