Hi whistlers,
I recently started a Celtic band here on my college campus, and in the whirlwind of music-finding and recording-sending and everything else, I’ve realized a flaw in my grand plan- I have no idea how to get chords for our guitar/piano/mandolin players on a lot of the songs I have. Is there a good search engine á la The Session for chords? My music theory for multi-note instruments is shaky at best…
Thanks for your help!
-CS
If you’ve got your music in ABC format, “ABCMus” http://www.norbeck.nu/abcmus/ will make a good stab at providing suitable chords for you. Alternatively try Google for “tune-name” Guitar tab or mandolin tab, banjo tab (even ukulele tab) or whichever … there are a lot of sites out there with listings of chord tabs. Once you’ve got the tab for one instrument it’ll be easy enough to transpose for any other 
If your backers need chords written out, then they aren’t ready to be in a band. They should work on learning how to figure out chords that will work, via lessons, listening, and practicing.
Also, chords for songs you are singing, or chords for the tunes you are playing?
While I wouldn’t go that far (depends on the nature/purpose of the band etc.), published chords for trad repertoire are so frequently inappropriate/unidiomatic that I’d want to do my own (or work with those capable of doing so) anyway.
And I bet you popped out of the womb a fully grown adult, too. I said starting a band, not ending one.
That looks cool. As long as we have a basic idea I’m sure we can correct any errors that appear. Many of our members, while accomplished musicians, are completely new to Celtic trad and so this will be a good way to introduce them to developing chords without having to dive in “cold turkey.”
Something I might add is trying to be alert to possibilities beyond the kind of major/minor triad/seventh stuff you get from the average ‘chorded’ setting. So, while experienced players of various trad string tunings may have a huge head start in interesting/idiomatic chord shapes, there are still loads of comparatively simple/obvious things you can try like sus4s, dominant elevenths (eg A E G B D or A G B D), ‘thirdless’ ninths (A E G B) etc. that I find effective on piano and frequently use in preference to more ‘classical’ harmonisations.
Here you are:
https://forums.chiffandfipple.com/t/a-beginners-guide-to-accompanying-a-session/71550/1
Yes, that was a good post. Thanks, Gerard, for the reminder and the link. 
I think the OP is really looking for written-out chords. Which, as Nico and Peter said, can be problematic given the creative nature of accompaniment. But if you change band to “learners group”, some combination using pre-written chords while also learning to move beyond them could be viable.
It was a good post indeed, it helped me a lot when I had the same problem as the OP.
As for prearranged chords there are two books called Ireland’s Best Tin Whistle Tunes 1 and 2 edited by Waltons that cover many traditional songs in a score form and with chords. I bought them in Dublin but I believe you can get them online as well.
Also, Tunepal occasionally retrieves ABC notations that include chords for the songs, you might want to try that as well, also it’s a damn good app.
And another thing; recently, while transcribing an amazing version of the London Lasses, I stumbled upon a software that can pick up quite effectively guitar chords from audio, so if you have the audio you can download a trial (full functional) version of the program here: http://riffstation.com/
Hope it helps.
EDIT: That said, as it has been stated before, usual accompaniment is richer than the one you might find on a book.