Here is a link with a few of the traditional versions of the lyrics as well as the sheet music. As you can see, it's from a bagpipe site. Whenever I think of what a bagpiper usually plays, it's something along the lines of "Wha Saw."
When sung as a round, starting a new group every two beats, it makes for a fun ending as the droning, steady rhythm gives way to an increasingly clear "bramble briar... bramble briar... bramble briar."
Here is a link to a midi file of the bagpipe version - chose the third medley down:
Not really finding any sung version on iTunes, etc. Anyone?