Been pondering this myself, and found this thread -

I, to, also often find their output too polished, and too mannered - their sound sometimes recalls the textures of group singing as found in Hollywood movies of the 1950s.

To my ears, Lou Killen tends to add a bit more grit to their sound - Tommy Makem seems to get subsumed within the polish of the wider Clancy sound

Full respect for them bringing the songs to wider ears, though