We have a few good depictions of the pre-invasion traditional Irish dress.

The costume consisted of the leine, a long saffron-dyed shirt reaching the knees with huge sleeves, the ionar, a short jacket, and the brat, the woolen mantle.

Here it is in 1521, the fellow in centre wearing leine and ionar, the fellow on the right with the brat worn as a cloak.



I think it's clear that in Ireland the brat evolved into a cloaklike garmet but in the Highlands it evolved into the breacan-an-feile. Both were wool, often patterned, and both often had a border either plain, fringed, or fur. It's the images of early fur-trimmed "belted plaids" which most clearly reveal the link to the brat.

There is that one early puzzling description of Highlanders wearing pleated/patched shirts daubed with pitch. It's unclear just what that was referring to.

Around 25 years ago, when I was regularly attending the Ren Faire, my costume was that "missing link" thing. I made a plaid wool plaid and trimmed it with fur. It could be worn as a cloak/brat or belted around the waist. My leine could be worn floor length or hiked up with my crois to be knee-length.