Did some googling and came across this:

However, as Jany Temime points out, "He dresses completely different from Albus because Aberforth owns a pub and Albus was a professor. The Dumbledores are Scottish, so the kilt was essential."

from http://www.wildaboutmovies.com/behin...DTHESCENES.php

It seems that, if this is a Welsh tartan, it was never intended to be. To my eye it looks like a nice plaid - and I also noticed the vest. Good look.