Dropkick Murphys are probably the best known celtic punk band I can think of.
The Pogues and The Dubliners aren't punk but awesome bands none the less. The Dubliners play proper drinking music. Get a load of friends together, drink more Guinness than any human should reasonably consume and you'll have the best night ever.
Yeah, if you like Flogging Molly you should check out the Dropkick Murphys...assuming you haven't already. The Pogues are indeed worth listening to as well.
On the more obscure side, Neck, Amadan, The Prodigals, and Siobhan, are all worth looking into as well.