From what I hear, a lot of Uileann pipers had chanters in flatter keys in the older days, whereas "the peoples key" is now deemed to be 'D', many uileann pipes would be pitched in 'C' or even B or B-Flat.

A lot of fiddlers would tune down to play with these chanters. As a result in some areas of Ireland, where the Pipes were prevalent, notably in Clare and East Galway there in a lot of fiddle tunes in keys that would be awkward on alot of instruments. This means a lot of Reels & Jigs played in C and F Major and Dm and Gm.

Most of these are awkward to back on the guitar without the aid of a trusty capo. However they are really nice keys to listen to in moderation. A long session all played in flatter keys can be a bit wearying after say the first 3 hours !