I've been a fan of the Tannahill Weavers for some time, but I've listened to their music with a fresh ear since taking up the whistle. So I set out to find some information on whistling in Scottish music.
The first stop was Nigel Gatherer's site, which features a short essay on the history of the whistle in Scotland and lots of Scottish tunes in ABC format.
Thanks to this Chiff and Fipple forums post I discovered the Folk Archive Resource North East (FARNE), "home of Northumbrian music online." Their music archive is like nothing I've seen online. ABC format? No way — fill in the search box and get back a scan of a historic manuscript! They have some interesting audio recordings as well. Read this article for a discussion of the history of flutes and whistles in Northumbrian music. Particularly interesting for me was the section on Billy Conroy:
A remarkable tin whistle player, Billy owned a small pet bird which would sit on the whistle, singing, whilst Billy played. His tin-whistles were always home-made, many from old bicycle pumps.
On this recording we hear whistle player Billy Conroy playing a seies of variations on one of Cumberland's most famous hunting songs - John Peel. Notice the way in which the melody becomes changed and increases in complexity with each repetion.
Update 2: Since I wrote this post I have interviewed Colin Melville on this subject.