Given that the (real) Scots were the Dal Riadans, who influenced the peoples east of the Druimn Albainn, and probably acquired things from them in return, and the Kingdom of Dal Riada originated in northern Ireland (possibly limited to the west by the Bann rivers) it would be unsurprising if the Scots did the same as the Irish. And as various writers (such as Cairney) suggest that the Erainnian Gaels might have had a Cruibhne origin too, it would be only natural if all these peoples dressed more or less the same. And given the sea-borne trade up and down the Celtic Sea, with intermarriages and population transfers, we shouldn't exclude the early Welsh from this.

So, it's quite likely that they all wore the leine and brat in some fashion: the leine was worn longer than it was in later times, namely to the ankle. It was probably girded with some device like a belt. I read somewhere (Ian Grimble?) that to ride one horse back, or to run when fighting, the leine was hitched up to the knee, or even above it. Could this have established a common style or wearing it, and securing the brat? It's very picturesque (Hollywood-style) to have a flowing cloak billowing out behind you, but I reckon it would be a bloody nuisance in a battle and I would expcet that most people would tie it around them so as not to snag in things - or give the enemy something to grab hold of.