There is a school of thought in old-time music that to be legit one must primarily learn tunes first-hand by listening to field recordings of guru fiddlers or the occasional clawhammer banjo player. There is also a preservationist attitude that to keep the music in its purest state it can only be passed down by ear and any other approach somehow dilutes it or takes away from the experience. While there is some truth in that, and I respect people with that level of dedication, for the rest of us we simply want to play these fun, hypnotic tunes by whatever means possible. For us, turning to a tunebook to get the jist of the melodic notes from the "dots" on a page is a perfectly acceptable method. Especially if we happen to play an instrument other than fiddle or clawhammer banjo where the blueprint is perhaps more forgiving or open to interpretation. (Why can't a saxophone player do old-time, for example?).
Which brings me to what I think is quite possibly the best overall collection of old-time tunes that I've come across: Dan Levenson's Old-Time Festival Tunes for Fiddle & Mandolin. (Note: there is a clawhammer banjo version of the book as well). 117 different tunes are covered; tunes that people actually play, written the way they play them! Granted, all jams are different...if you traveled around the USA I'm sure you'd find that each community of jammers has their idiosyncrasies, core tunes, and ways of playing them. But I'm quite certain that a good percentage of the tunes in this book are going to come up at any regional session, and if you play these versions, smile and enjoy yourself you'll fit in just fine.
Old-Time Festival Tunes for Fiddle & Mandolin is a spiral-bound book. Tunes are presented in standard notation and very legible tab; a plus for me as I am primarily a tab reader although I can also muddle through standard notation. (The tab is intended for the mandolin but also works for tenor banjo tuned GDAE, which is what I play. Note I said tenor banjo tuned GDAE and not Irish tenor banjo although I do also play Irish tunes on my tenor banjo. Tunes like St. Anne's Reel and Staten Island Hornpipe. Hey wait a minute those tunes are in this book! What does that make them? What does that make me? But I digress.)
There's actually two versions of notation for each selection - a basic version to help you get the melody under your fingers and an advanced ornamental version for when you're ready to tackle a more challenging take on the tune. The tab version is different still - more than the basic notation but not as complex as the advanced notation. So if you can read both tab and sheet music then you actually have 3 written variations to draw from. The suggested chords are printed above the standard notation line. Concerning chord selection, Levenson explains that chords aren't as fixed as some might think (I agree) and admits to using ones that he thinks serve the tune the best even if the old timers might not have had these in their bag of tricks. However if you find that other chords work better then he encourages you to by all means use those.
Also included are two CDs where each tune is played through at a moderate speed. These CD versions are similar enough to the written versions to let you hear how the tune sounds but they are not a note-for-note playing of what has been written. So, with the recordings you actually have a 4th version of each tune that you can work on by ear. You'll appreciate this later even if you don't at first. You don't have to stop there though. Go on YouTube, poke around the web, compare to other written sources, and use this book as a supplement to what you find. Absorb as much as you can.
Ultimately the goal of a book like this is just to get you off the ground and playing this music your own way - a launch pad towards personal expression and/or playing with others. It shouldn't be taken too literally. There is no one definitive way to play any of these tunes. The right way is the way you like it. So get out that Irish mandolin and learn some of these old-time festival tunes!