The creideamh si was, for all intents and purposes, a component of Gaelic custom and tradition alongside the Christian religion. As to its relation, specifically to Celtic Christianity, I can't comment much. I could suggest that you ask the question on the "Celtic Christianity" board; they'd likely be able to provide some insight on how they, as modern CC's approach the aes sidhe.
Truth in our hearts, Strength in our arms, Fulfillment in our tongues.