The first way in which paganism is present in the Secret of Kells is through Ashleigh, the fairy Brendan befriends in the forrest. Although a belief in fairies was a part of the pagan religions the Christian church was trying to stamp out in Ireland at this time, Ashleigh is shown in a positive light in the movie and helps Brendan. This positive image of Ashleigh could be a result of the fact that she is almost a personification of the forrest, which was necessary for monks to create ink so that they could illuminate manuscripts. This use of plants from the forrest, which monks like Challach considered dangerous and wild, to further the spread of Christianity is an example of turning darkness into light, a common refrain in the movie. Even though Ashleigh is friends with Brendan when he is a child, when he comes back to Kells as an adult she will no longer reveal herself in her human form in front of him, which could show that he has succeeded in spreading Christianity in Ireland through the Book of Kells and that paganism, although still present, has been mostly suppressed.
Paganism is also presented in the film through Crom Cruach, who at first seems like the villain of the story. He (possibly symbolizing paganism in Ireland) is responsible for the spread of darkness. However, in the end it turns out that Crom Cruach, or at least a part of him, is necessary for Brendan to complete his illumination of the Book of Kells. This is another example of the theme of turning darkness into light, but it also implies that the spread of light (Christianity) is impossible without the incorporation of at least a little darkness. I took the use of Crom Cruach’s eye to complete the manuscript as a metaphor for how it was necessary for the church to allow certain aspects of local pagan religions to be incorporated into Christianity in order for the religion to spread successfully. Overall, the characters of Ashleigh and Crom Cruach showed how complicated the relationship between local Irish beliefs and the church were at the time of the Book of Kells.