[Anguirel]

In the Scottish lore I know, cailleachs alternate between the crone and the maiden aspects, becoming beautiful if they are wooed and kissed. They are tied closely to the land and represent its sovereignty and fertility. The Loathly Lady Ragnell who became the wife of Sir Gawain was based on such legend. I hadn't heard of a connection with water, though I'm sure you're right.

However, Tolkien seems to have far preferred the Saxon legend to Celtic stories. I suppose, though, that that would not prohibit him from borrowing from them; the Elves often seem not unlike Welsh or Irish figures, living under the hills or in the woods.

Goldberry herself, from the little we see of her, seems intangible, elusive, but worthy of worship and Tom's gifts of lilies and the Barrow jewelery. But she is constant and unchanging at the same time. "Tom has his house to mind, and Goldberry is waiting."

Her fair looks recall Rhiannon, the fey mother of Pryderi in the Mabinogion. Beyond that, I can't get much further. She is related in her influences to Galadriel, and yet wholly different; in a way above the Lady of Lothlorien, greatest of the Elves though she be. She receives adoration without extracting it. She remains secretive and mysterious.