Scottish Gaels wore the Leine like their Irish counterparts in the middle ages. It is gaelic for shirt/tunic and is essentially a tunic with pleats in the back. They were usually saffron in color and went to about the knee. Trousers were not usually worn with them for most of the era, they often went barelegged. The brat was a large tartan plaid that was wrapped around them, more like a cloak than the belted plaid it will become to be. Footwear would consist of currans or pampooties which are deerskin moccasins.

For more info, I'd recommend Matt Newsome's website Albanach.org, it has tons of accurate info on the early histories of the kilt and Scottish gaelic clothing.