To widen the scenen a little further Ortigueira in Galicia always has an awesome lineup of celtic bands and is FREE!!

http://www.festivaldeortigueira.com/english.php?lang=ing

Another festival, more roots based but large, in fact VERY BIG is Wychwood which started for the first time last year. Some big name artists (Billy Bragg, Saw Doctors, Amadou and Mariam ) and on the Sunday night a collaboration with Eliza Carthy and Salsa Celtica which will probably be worth the price if admission alone. All one one site (Cheltenham racecourse) so good camping facilities.

My daughter works full time for the organisation so I declare an interest.

http://www.wychwoodfestival.com/

My own favourite is the National, followed by Warwick of the larger festivals.