I've just had a look at the 2007 Dublin Irish Festival entertainment lineup. Tucked in amongst the "Celtic rock," I found some pretty good reasons to attend, including the following out-of-town acts:
There are also some local bands I like. My teacher's band, The Kells, who played a great house concert in April will be performing. Occasionally-local Changeling is a husband-and-wife duo featuring the fiddle playing of Deborah Clark Colón. Aisling reformed last year after significant personnel changes; I haven't heard them in their current incarnation, but they were quite good in the past.
An intriguing-sounding group I'm unfamiliar with is the Armagh Rhymers, from, of course, Co. Armagh, who do folk theatre from the mummer tradition.
There's also a great deal of utter dreck on the bill. I'll elect to follow the "if you can't say anything nice about someone, say nothing at all" rule here, and simply write this off as the price of having a festival which can bring good acts in from overseas.
But since the festival generally has at least four stages running concurrently, plus session tents and other stuff, you can generally find something good to hear or see.