December 6th is another 'First Thursday', when galleries and other cultural spaces stay open late, frequently with launches or special events. It's run by Temple Bar Cultural Trust and details of this month's First Thursday are here.
At 5:30 pm the Christmas lights will be switched on in Temple Bar's Old City, with mulled wine and mince pies. (Actually the lights seem to have been on for some time already so I'm not quite sure what TBCT have in mind... but hey, mulled wine and mince pies.) Then at 6 pm First Thursday proper kicks off; officially it continues until 8 pm but sometimes individual venues are still open after that.
Worth a look: Project Arts Centre for their tour of their latest exhibition; Little Green Street Gallery launch a new exhibition (and their launches are fun); Science Gallery; White Lady Art, a new gallery just recently opened.