ancient history is a fuck. all we have are clues and we’re left to connect the dots. but my personal theory is that the Celts originated in Gaul, colonized a bit in the east and west (hence their presence in current day austria and the west coast of europe. then as a result of the romans terrorizing them, migrated north until they colonized terf island and pushed the Picts north. then when the romans came to the isles the Celts were pushed north and mixed with the Picts (who knows how long the Picts were there for, they might have been only a few centuries removed from the Celts)