it's the same as this animated-GIF of the supposed de-evolution of the human face back to its ape-origins
it just doesn't work, there's glitches in the desperate-hope of ancestral linearity, there's obvious missing links and irregularities in the single path back to our supposed ancestors... much like Proto-Indo-European into Proto-Celtic into the supposed Celtic trade language of the stone-iron age to the modern Atlantic-coast dialects of the title of this post.
Proto-Celtic (let alone its supposed father-tongue Proto-Indo-European) just doesn't sound related in any way shape or form like there's an unspoken missing link language or people who came from the West, a real sea-faring people and or island that no longer exits, was destroyed, became extinct via Natural Cataclysm, Space Debris or Ancient Atlantian War.
Maybe something like a far-more-ancient westerly-language (unrelated to all the easterly European-migration languages) that brought with it the desire to make (and the abilitiy to move) stone monuments and worship the movements of the stars. There have been many tales in myth and ancestry of a Westerly People, like the Aman in Tolkien's SILMARILION.