"It's been a while since I read Barber's book, but didn't she reference older textiles with a similar weave (twill) from somewhere in the Middle East, IIRC Anatolia? But the color of the cloth wasn't plaid." She compared them directly to the Celtic fabrics found at Halstadt. They are exactly like those at Haldstat, material, mfg technique, color, weave style and etc...1,000 years and 4,000 miles apart.
I think all the Caucasian folks out in that region were refugees from the Black Sea flood (Noah's Flood?) in 5,600BC.
It's not unreasonable. The Celts that settled Scotland, Ireland, Wales, and NW France probably came from Switzerland and Austria when they were pushed out by invading hordes from the SE.