A piece of tartan cloth found on the ship Osebergskom. The Huldremose woman who lived in "the Iron Age" wore a plaid gown. Wikipedia says the find is around 340 AD at the latest. (And now no one can argue against Vikings wearing plaids.
Herjolvsnes no 38
Some Clothing of the Middle Ages -- Kyrtles/Cotes/Tunics/Gowns -- Herjolfsnes 38