Cartimandua and Brigit

The key paragraph here, where Koch and Carey suggest that Brigit and Briganti are correspondents, and that Brigit arose from Brigantes in the Kildare area, and even that she had a "mortal high priestess" there is all speculation, unproven and as far as I know unsupported, but this does help explain why so many people assume it to be true.

from The Celtic Heroic Age by Koch and Carey, 4th edition, 2003.