I know of two tunes for it, both, I would say almost equally used and popular in Scotland, but the one used by Chris Thlle and Nickel Creek and guitar-picking guest (and other American recordings on Youtube) seems quite different from either of these.

Here's kenneth McKellar: http://www.youtube.com/watch?v=TzT4g9ahLvU&feature=related

And the other tune (nicely played on ukelele): http://www.youtube.com/watch?v=rgvBq9XuydE&feature=related