The real stuff is excellent, and so is some of the modernists. I even have a soft spot for some of the punk bands with clear celtic roots like the Pogues or to a lesser extent Great Big Sea and the Dropkick Murphies.
What I'm not a big fan of is the "music hall" variety, like the Irish Rovers, or the worst of Makem and Clancy (to be fair, much of M&C is pretty good)