My page on this song is updated now. I added information provided to me by John McLoughlin, reorganized the page, and added another illustration. Regarding the interest in using elements of mythology and nature, I am surprised that anyone would think they would exclude references to Christianity, Judaism, or any other religion in the same song. Here is something John McLoughlin wrote to me about the author of these lyrics, Joseph Campbell:
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Joseph Campbell (1879-1944) came from a fairly well to do Belfast family of road builders but he gave it all up to be a poet. He had a mystical nature (as evident in Gartan). In 1906 he wrote, "all things on earth to me are known, for I have the gift of the Murrain stone" ( a "magic" hollow stone from which cattle would be made to drink in the hope of preventing them catching the deadly disease, murrain).

The song was first published in 1904 in "The Songs of Uladh [Ulster]" by Herbert Hughes and Joseph Campbell. Both were from Belfast, Hughes being a Protestant (Methodist) and Campbell a Catholic. Hughes collected the trad melody in Donegal (of which I am a native) the previous year and Campbell wrote the lyrics.
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Myth, fantasy, and nature themes can be used and appreciated by Christians just as much as by anyone else. After all, C. S. Lewis, the Christian apologist, wrote the Chronicles Of Narnia.