Other good sources of fiddle tunes are M.M. Cole's
One Thousand Fiddle Tunes and
Ryan's Mammoth Collection (the latter available in PDF form here:
http://violinsheetmusic.org/collections/ryans-mammoth-collection.pdf ). I think those two are related in some way—there's sizable overlap and the typesetting is the same. Lots more Celtic and even American fiddle tunes are available from these sites:
The Session:
http://thesession.org/
With scores, MIDIs and ABCs.
Ceolas: The Fiddler's Companion:
http://www.ceolas.org/tunes/fc/
ABCs only.
Note: for Ceolas ABCs, add the line "X:1" before the rest of the text. Every ABC tune has to start with an X line (tune number).
ABC is a text-only representation for simple music. It can be used to generate scores and MIDI files using ABC programs such as the free, cross-platform suite EasyABC or the online ABC converter at Mandolintab.net. These programs will also transpose the tunes for you during the generation process. If you hear a clip of a fiddle tune you want to learn, there's a good chance one of these sites will have a version (or five) in ABC form, from which you can create a score in the key you want.
Online ABC converter:
http://www.mandolintab.net/abcconverter.php
EasyABC:
http://www.nilsliberg.se/ksp/easyabc/
Perhaps this is obvious, but if one has a guitar background, an easy way to transpose from the bottom-G fiddle range to reentrant C-tuning is to play the music as if you were playing guitar (or baritone uke). If you play low-G instead, the tune should fit in C-tuning without transposition.