Ireland is awash with festivals all summer long, http://fleadhcheoil.ie/ The All Ireland Fleadh is a week long festival of Irish traditional Music (and lots more besides) running from 10th - 17thin Sligo. Lots of concerts, sessions, and classes/workshops to suit all abilities. It gets very busy on the weekend when the competitions get under way.
The Ballyshannon folk festival will usually happen about the beginning of August, and is very close by. Website not up and running yet for this years event, but they usually have big named acts. Check this later or see what was there last year. http://ballyshannonfolkfestival.com/