Two sources that have been invaluable to me are:

John Doyle's Mad For Trad guitar tutorial on CD-Rom. Doyle uses Dropped-D for backup and it will stretch your left hand out a bit. Even if your not into Doyle's style (I am but the Kilkelly book below helped me develop my own style) it will give you a good primer in chord progressions and substitutions as he accompanies John Williams (concertina). Lots of good jigs an reels and fun listening. They seem to have turned this into an online lesson at TradIrishmusic.com with Niall Vallely on concertina that I'm not familiar with.

Frank Kilkelly's Accompanying Irish Music on Guitar (Book and CD) deals with a bunch of tunings and styles of some great players (really demystified things for me) and features Kilkelly backing individual players who are all quite good. This could be good for someone looking for direction and again lots of great tunes and playing.

I fell in with an Irish/Scottish trad session about 10 years ago and and these two items gave me a big leg up.