Well, it's been a while since I updated the master list in the first post, but you'll find a thread here on celtic music. It's not all Celtic rock, but it has some stuff worth investigating. I'll try to get it up to date tomorrow - check back later.
Just off the top of my head... check out Seven Nations, The Pogues, The Prodigals, Bad Haggis, The Young Dubliners, The American Wake, and The Saw Doctors.