1594, a documentation by an Irish author. I believe the book was "Red Hugh O'Donnell". It's in Irish Gaelic and I think it mentions a small group of Highlanders and their attire.

Since we're on this subject, I'm going to flag an old video by the Discovery Network that used to air on "TLC" back in the early 1990's. This aired a couple of years before "Braveheart" and I think it might be one of the sources for the reasons as to why the Scot's are in some strangely worn belted plaid.

Here's the link "Ancient Warriors: Highlanders" 1 2 3