To the best of my restricted knowledge a 'fleadh' is a 'feast' (festival), nothing else. In combination with another word, 'Fleadh Cheoil', you may safely expect traditional Irish music. Since 'Fleadh Cheoil' has sometimes been shortened to 'Fleadh' (as if there would be no other feast possible) people have started expecting a 'Fleadh Cheoil' when they saw the word 'fleadh'. Look e.g. at 13th Galway Film Fleadh to see how wrong such an expectation can be. All in all, you should be glad that the London Fleadh is still about music of some kind and not about something completely different which would still be covered by the word's meaning.

On the other hand, when I see the list of bands and musicians and with the additional information that it once was about 'Celtic' music, I can understand your frustration.

Wolfgang