There's a wealth of lead lines available for traditional tunes out on the net, for example, the Lewes Tunebook, The Session, etc.

What is harder to pin down is the "standard" harmonic underpinning of the tunes - the chords - which one can play in a session, and which are the accepted chords. There are many options for chord sequences for such tunes and, while some are intuitive, some are not so. I have a good ear and am fairly good at anticipating chord changes on guitar when hearing a session tune for the first time - but I don't know, unless someone else more familiar with the tune is also playing it - is that my changes are completely correct.

Anyone know of any source(s) for chords for the tunes?