Where would I go looking to find the right changes for tunes? Everywhere I go I seem to find strictly melody.
I know how to harmonize a melody, but I want to make sure if I show up at a session looking to play accompaniment on a few tunes that I play the chords people are expecting to hear & not just the chords I assumed were right.
I remember reading somewhere recently that if I don’t know the tune, I shouldn’t play, so give me some advice on where to go to get the rest of the story.
I don’t think you will find what you’re looking for in print. I’ve been playing melody only for yrs, so dots/sessions/cds worked fine for me. But now that I’m moving into accompaniment there’s not nearly as much to find. It seems to be more loosely defined than, say, bluegrass. More interpretive, if you will. It makes the tunes more interesting because the chord progressions are more bendable. I think it also requires a better understanding of the melody.
Just my 2 cents.
Your best bet may be to ask at the session, as different people play the same tunes so many different ways and the chords are often different with different version. I went to a lot of work a few years ago to find, verify and compile the chord changes to a whole lot of tunes in the Celtic and Old Time traditions, and I can tell you, even the very people you get the chords from are gonna play it differently later. 
The Waltons ‘110 Ireland’s Best Session Tunes’ books have melody in notation and accompaniment chords.
They have produced a third, so the library contains 330 tunes if you get all 3 volumes. They are available with CD and are a pretty good resource - as are the equivalent ‘Ireland’s Best Tin Whistle Tunes’ books/Cds - which also include chords.
These books do have most of the common session tunes and some interesting ones which one hears less often.
a few of the versions aren’t always quite as most widely played, but I’ve found them easily good enough for me to not disgrace myself when I join in sessions during Irish vacations.
Of all the books I have, these have probably been the most useful overall.
http://www.bigwhistle.co.uk/shop_results.asp?search=1&subcat=11&keyword=110&GO=GO+�
http://waltons.ie/waltonsshop/product_info.php?products_id=11057
I’m going to try a blended approach then. I’m going to hit the local session next week (just to observe & get advice at first), and I’ve just found a dealer this side of the Atlantic to ship me Ireland’s Best Session Tunes, Volume 1. Now I’ve just got to decide which tuning I’m going to use & get learning.