The Celts did absorb some of the older faith into their own, taking basic deities and complicating them with myths. There was a tradition whereby the King had to reaffirm his place by first mating with a sacred white mare and then cooking and eating her flesh and bathing in the water that was left after the horse's bones were boiled.
The mare was the symbol of the Goddess and the King had to become Her consort in order to rule. The Mother GOddess was the conduit through which the souls of the dead could pass into the Summerlands and be reborn again.
"The Religion of the Ancient Celts" by J. A. MacCulloch (
"The encyclopedia of Celtic mythology and folklore" by Patricia Monaghan (2004)