Folkloristic Survivals
Nagy has noted the Gaelic oral tradition has been remarkably conservative. The fact that we have tales in existence which were still being told in the 19th century in almost exactly the same form as they exist in ancient manuscripts leads to the strong probability that much of what the monks recorded was considerably older. Though the Christian interpolations in some of these tales are very obvious, many of them read like afterthoughts or footnotes to the main body of the tales, which most likely preserve traditions far older than the manuscripts themselves.
Mythology based on (though, not identical to) the pre-Christian traditions was still common place knowledge in Celtic-speaking cultures in the 19th century. In the Celtic Revival, such survivals were collected and edited, thus becoming a literary tradition which in turn influenced modern mainstream "Celticity".
Various rituals involving acts of pilgrimage to sites such as hills and sacred wells which are believed to have curative or otherwise beneficial properties are still performed. Based on evidence from the European continent, various figures which are still known in folklore in the Celtic countries up to today, or who take part in post-Christian mythology, are known to have also been worshiped in those areas that did not have records before Christianity.
In Twilight of the Celtic Gods (1996), Clarke and Roberts describe a number of particularly conservative folkloristic traditions in remote rural areas of Great Britain, including the Peak District and Yorkshire Dales, including claims of surviving pre-Christian Celtic traditions of veneration of stones, trees and bodies of water.
Read more about this topic: Celtic Polytheism