I play fiddle tunes on cello. Occasionally I learn by ear or play from memory, but usually rely on sheet music as an aid in learning a tune, working out and/or playing a backup part, or performing a tune. Often fiddle sheet music, written in treble clef, is sufficient, and I simply play the tune an octave or two down from where it is written. Sometimes, though, it is easier to play from music written in the bass clef.
This is not a comprehensive list of cello tunes; it is merely my personal collection of copyright free tunes that I enjoy playing.