Some - but not many - of the tunes you'll find via abcnotation.com also include chords. For example a tune on that first Clannad album:
BRIAN BORU'S MARCH
You'll see the chords (Am, G...) above the stave and also in the abc.

It's not much, but it's a start! I use the chords, or parts of them, for an accompaniment on a concertina. abc is also easy to change the key.

I don't know whether you'll find the kind of multi-instrument midi files you seek. But good luck.