For the Welsh reflex of Roman castrum, see caer.

In Irish mythology, Caer Ibormeith was a daughter of Prince Ethal Anbuail of Sid Uamuin in Connacht. Every alternate Samhain she would change into a swan, in which form she would remain for a year before becoming human again the following Samhain. She eventually married Aengus, but first he had to pick her out, in swan form, from a group of one hundred and fifty other swans at Loch Bel Dragon (Now Lough Muskry in the Galtees.) Having chosen correctly, he turned into a swan himself and they flew away, singing beautiful music that put all its listeners asleep for three days and nights. With Aengus, Caer was the foster-mother of Diarmuid.