Irish traditional dance music is perfect for the whistle so try these sites: http://www.ceolas.org/tunes/ http://home1.swipnet.se/~w-11382/tunes/tunes.htmThe ABC versions are excellent and much quicker to download than photo-type formats (gifs, jpgs etc) and the software you need is cheapo downloadable shareware (you'll find links at the sites that offer ABC versions.)
I you want to know what these tunes can sound like go to online CD stores (eg via Chip & Fipple http://www.chiffandfipple.com/table2.html)
and listen to snatches of Joannie Madden or Mary Bergin.