If it is wool you could make a kilt!

Make blocks with black.

Major part of a Christmas tree skirt? Pieced with green and off white it would be lovely- classic and very old fashioned.

Put you a Scottish boy or dog or both in a center piece. Use coordinating fabric for squares and then use the rest for backing. I do not know how much you have but, even if you have to piece your back it will be beautiful.

I agree with using it as a whole piece and appliqueing solid forms on it. Something that you enjoy - animals or trees and houses or just some shapes. Depending on how many and how far spaced, you could simply echo quilt around them and maybe add a few straight lines following the plaid. If it is wool, you could simply use a flannel backing without batting and it would make a toasty throw for the couch. If you are still learning, this might be a simple and yet good way for you to practice and have a nice finished product. I am not a pre-washer, but I would launder all pieces first to work out the shrinking. (Being Scottish and this fabric coming from Scotland, I know this is a fine piece. It's beautiful and I'm sure it feels so very fine.)

Oh my, this just screams shawl to me. You could fringe all 4 sides and fold diagonally for a shawl. Easy peasy.

My first thought was a kilt of course but my second thought was throw pillows. Make one for her using this as the focus fabric for simple block or if you are an applique person add a thistle with the plaid or "plaaid" as a Scot would say. You can also let it sit and brew for a while..... some good idea will come to you.

Perfect idea!

I have 5 yards of Montrose tartan so I am in the same boat. For now it is safely tucked away from moths. The idea of applique is really good.