Over the course of the holiday season I got all three seasons of the original Star Trek on DVD. Needless to say, I spent a lot of time in front of the TV. Kilts in Star Trek have been mentioned before, but here's a full rundown, as far as I've seen, with an oddity I noticed.

In "Is There in Truth No Beauty", Scotty wears his kilt at the dinner for Lawrence Marvick and Dr. Jones. It's a red tartan (if it is a tartan), and he's also wearing red kilt hose.

In "By Any Other Name", Scotty doesn't actually wear his kilt, but it's seen hanging on the wall of his quarters in the scene where he gets the Kelvin intruder drunk as a klingon lord. His bagpipes are also seen, in the alcove that's the vanity in all the other quarters sets seen.

In "The Savage Curtain", Scotty again dons the kilt, to recieve "President Lincoln," but this time it's not red, it's white and black. Perhaps a dress tartan?

These are the only kilt occurences I've found in the Original Series. I don't know about the other series.

Just as an aside, James Doohan has said he based Scotty's accent off of a radio operator from Aberdeen he met during WWII.