I don't think there are many who doubt that the Clancy brothers paved the way for Irish music in other forms. But there were some things that bothered people about them that I think were legitimate.

The idea of pretending a song was Irish was one of those things. Indeed, the Dubliners recorded The Wild Rover before the Clancys did, but made it clear it was a song Luke Kelly had picked up in England. The Dubliners weren't ashamed of Luke's apprenticeships with Ewan MacColl, and the influence of both English and Scottish material on his repertoire. The Clancys pretended everything they played was Irish. There was also a general hokiness about Clancy shows that bothered some people, with everything being very scripted. There was the fact that, once they became big, they hired some of the best musicians in Ireland to tour with them (eg. Nollaig Casey and Arty McGlynn), but hid them behind the curtains so nobody saw them. That way, it looked like they were really playing their instruments. These are all minor things, of course, but they added up and annoyed a lot of folks.

Then there was a feeling among many Irish people that the Clancys were playing music aimed at the American market, and that they were in fact Americans, "returned Yanks," who had made millions of dollars simply by marketing their Irishness. They felt it unfair that some people could get rich by being "professional Irishmen" while many other Irishmen were poor. So that added to it.

However, they did gain a level of acceptance on their own terms in Ireland, and of course were hugely influential on the whole "ballad group" phenomenon. Then, in the following generations, another development occurred: the generally rudimentary nature of the musicianship in ballad groups turned off many young people to Irish music, while the stiff classical arrangements of the Chieftains didn't help either. Bono for one has spoken about this; they learned about Irish music in school, but hated it. So there are a number of different reasons why people don't like the Clancy brothers.

But the bottom line is, even if you don't like their music, if you like what came after (eg. Planxty, The Bothy Band, Altan), then you owe them a debt.