I have just heard of the HUGE concert in Bethlehem, PA every summer - Musikfest. There are over 300 artists lined up - 95% of which are FREE. A fair number of the artists are Celtic. There are 15 different stages for 10 days/nights.
Musikfest is from August 6th - 15th.
Celtic (free) events: Kindred Enter the Haggis The O'Grady McManus School of Irish Dance Brock McGuire Band Tempest Devilish Merry Wolfstone The Young Dubliners ..and more
And not Celtic, but famous (ticketed) events: Alice Cooper Crosby, Stills and Nash Peter Frampton Creedence Clearwater Revisited (CCR members reformed)