The Breton folk song Gwerz Santes Enori has survived for centuries as an oral tradition combining storytelling with music. Its story, quoting Wikipedia's summary, begins like this:
The poem's story concerns the youngest of a king's three daughters (the king of Brest, or Brittany, or Spain, depending on the version), who sacrifices herself when her father is bitten by a snake. Only a virgin breast can save him, and Enori, the youngest, neglected daughter, offers herself up after her two sisters refuse. When she goes to help him a snake jumps onto one of her breasts, and her father cuts off the breast, after which he is miraculously cured (Mary-Ann Constantine identifies this as a "Celtic theme"); the daughter is rewarded by an angel who brings her a golden breast, and she gets a husband as well.
Which element of this story is said to be a "Celtic theme"? The cited source goes to a Google Books version of Celtic Culture: A-Celti edited by John T. Koch, but I tried searching it for the phrase "Celtic theme" and didn't find anything relevant.