TGBHF is the only U2 song that has lyrics written by someone outside of the band. They were written by Salman Rushdie in his book of the same name, and were presented as lyrics in the book. The novel's about a great rock musician and his life long love for a beautiful singer. There are obvious (and intentional) parallels between this story and the mythical story of Orpheus. Here's a bit about him:
"Orpheus was the son of Calliope and either Oeagrus or Apollo. He was the greatest musician and poet of Greek myth, whose songs could charm wild beasts and coax even rocks and trees into movement. He was one of the Argonauts, and when the Argo had to pass the island of the Sirens, it was Orpheus' music which prevented the crew from being lured to destruction.
When Orpheus' wife, Eurydice, was killed by the bite of a serpent, he went down to the underworld to bring her back. His songs were so beautiful that Hades finally agreed to allow Eurydice to return to the world of the living. However, Orpheus had to meet one condition: he must not look back as he was conducting her to the surface. Just before the pair reached the upper world, Orpheus looked back, and Eurydice slipped back into the netherworld once again."
The lyrics in Rushdie's novel are just some of several created for fictional songs "written" by the main character. The book, in proofs, was sent to Bono before publication as Rushdie was concerned that he get his portrayal of the life of a rock star accurate, and he wanted Bono's feedback. Bono was struck by those particular lyrics and decided to put them to music.
By the way, in the novel a massive earthquake occurs and the beautiful singer is swallowed up. So when the songwriter says that "what he worshipped stole his love away/It was the ground beneath her feet", he means it literally. It's a good example of Rushdie's rather quirky sense of humour.
Hope that helps.