For the British Isles: If you want primary resources, then Vita Brigitae, and the First Synod of Saint Patrick give some 6-7th century perspectives on Druids and Christians, though don't expect too much of a fight. You could also include Muirchu's Life of Patrick but it's written much later (8th century).

Modern history-wise, Ronald Hutton has a very readable The Pagan Religions of the Ancient British Isles but that covers from about 30,000BC so most of the book isn't about the clash -although his last few chapters deal with the interaction between the two sides (hint: not really a clash, and we have no literary sources at all for the Romano-British period). Barry Cunliffe is the other one I'd have a look at (The Celts) but again, there's not very much on clashing, although again, he is mostly Gaul/Britain focused - I'd probably take Hutton over him on this because Hutton writes more on it. Cunliffe also did the Druids: A Very Short Introduction which is a bare-minimum take on it if you can't afford the others.

Religon-wise, both exhibit a fair amount of syncretism with Christianity, so there appears to be not much of a clash, in fact, most of the clashes that I've read about are Roman vs Celt, rather than Christianity vs Celt. Most of our knowledge about the Celtic traditions survive because the (Irish) Christians wrote them down, mostly because, as Cunliffe says, they were interested in making connections with Christianity by looking at the universal truths contained in the pagan sagas.

The idea that there was some huge 'clash of civilizations' during the conversion of Celtic speaking peoples is a pretty misleading way of looking at this phenomenon. In Ireland, there was evidently some initial opposition to conversion by the spiritual and to an extent, military elite, but Christianity was eventually molded around indigenous belief and society. The druids became clergy (the 'Celtic tonsure' was probably the same style of hair worn by the druids) and pagan figures and places were coarsely Christianized.

You have to understand that pagan peoples, expect maybe the druids, had no real philosophical attachment to their faith, but they were attached to the magical figures and places that could help them in their life. When they became Christianized, it wasn't such a big deal for the majority of people. Their local gods became Christian saints (see the transformation of the goddess Brigit into the Saint Brigid) and their healing springs and magic wells became holy sites.

The role of magic in the Irish (and generally temperate European) perception of religion is also important to consider. Christianity was a new and more powerful form of magical manipulation, with a clear dogma and text which contained demonstrable examples of the new god's power. Christianity was consequentially used by new practitioners as healing magic, a form of divination and most importantly battle magic, which I think explains its rapid popularity with the indigenous warrior-aristocracy.

Anyways, I think you're looking at this topic the wrong way. Christianity didn't root out local traditions; in fact, it adopted and preserved them in very significant ways. We can learn most about pre-Christian Irish belief by learning about Christian popular belief in post-Conversion Ireland, simply because they synchronized both belief structures and wrote stuff down, which the indigenous people hadn't really done before. There was definitely antagonism on the personal scale, but there was no organized pagan resistance against conversion.