Could it be that Celtic Thunder is polished, professional and successful, that they create such ire among people? After all they perform using material both modern and traditional.
As a poor example "Ride on" has been recorded by Christy Moore and Caltic Thunder. Is one performance supposed to be more authentic than the other?
How do you define authentic anyway.
Some liked the Pogues, others detested them. Celtic Thunder must be liked by many because they are big and successful. There is no law saying you have to hate them because they have made it. You may criticise them as much as you like, but is it valid, or coloured by a touch of envy?

By what set of rules do you judge them? Is it not a good thing that they bring folk music to a wider audience?