"The Song of Wandering Aengus" by W.B. Yeats is based on a story about a Celtic god, Aengus Óg, who is often associated with young men, love and poetry.
There are several tales about this god in various Celtic myth cycles, but the one from which Yeats drew his inspiration is probably the one in which Aengus sees in a dream a beautiful maiden, Caer Ibormaith, the daughter of Ethal, and falls in love with her. He and his family spend three years seeking the woman of his dreams. It turns out that she is one of 150 girls who are imprisoned by silver chains attaching them to the lake Dragon's Mouth and condemned to spend every other year in the form of swans. Ethal, mother of Caer Ibormaith, promises that Aengus can marry her daughter if he can correctly pick her out from the 150 swans. Aengus does so, and they both happily fly off in swan form. Although there are many variants of this legend, the story of the fish is actually not part of them and was imported by Yeats from other elements of Celtic mythology.
There are two basic connections of this poem with Christianity. First, in one Irish tale, a foster daughter of Aegnus is converted to Christianity by St. Patrick, and despite Aegnus' best efforts, leaves the world and company of the sidhe and dies. The second element of Irish history connecting the name Aengus with Christianity is St. Aegnus, an eighth-century Irish monk and author of Feliré, or Festology of the Saints, who spent many years in the wilderness.
What both the saint and the Celtic god have in common is the notion of quest, of wandering through the wilderness seeking some transcendent goal.