http://www.itma.ie/home/leaf3a.htm Visit the website of the Dublin-based "Irish Traditional Music Archive". You'll find here some some excellent introductory material for beginners or newcomers to Irish music
http://www.standingstones.com/modeharm.html#renmode Ionian? Mixolydian? Dorian ? Aeolian? What's all this?
A beginner's guide to modal harmony, with fairly detailed historical background