Starting from the back end of the city centre. Caledonia, Pilgrim, Ye Crack, Grapes (Knight St), Hanahhs Bar (check if theres a band on), Salt Dog Slims (Nice hot dogs), Pouge Mahones, Santos Chupitos, the hidden bar next to Santos Chupito.(Jungle Juice cocktail), The Swan. You can always try Django's Riff and Bar CaVa Although there not really rock bars.
Then there's the Krazy House but that's more of a club. Or The Camp And Furnace-Try and catch rockaoke Vs Food Slam, some tasty grub (Piggy Smalls) and karaoke with a live band. Only problem with he Camp is it's a bit out of the way, maybe a 10/15 min walk (if you don't get lost!) to the nearest bars, there are always taxis though.