I would have thought the great big clue to the area’s celtic/gaelic origins would have been in its very name.
Wales is “Pays de Galles” in French, the language of Scotland is Gallic, the Irish are Gaels, I’m no linguistic expert but there seems to be a bit of a link. Plus the fact that like Wales, Ireland, Cornwall, the Isle of Man, the Scottish highlands an islands, Brittany etc. it’s another one of those rugged little enclaves of northwestern Europe where the Celts all seemed to end up.
You know for such a tough bunch of guys the Celts sure let themselves be pushed out into the fringes by whoever happened to show up later in their homelands.
When the briefly-toughest guys settle into the comfortable bottomlands, they grow soft in just a few generations. Meanwhile, the mountaineers get more and more counterproductively tough, until you can't do anything but kill them. (Afghans, for example.)
The Persians recognized this pattern in the early Classical age, 5th century B.C. or so. Once your warrior class moves out of the high steppes and gets comfortable, it's all over.
The closest part of France to America was Armorica. No doubt an academic would just consider that coincidental.