Droichead Beag in Dingle, Lobby in Cork, Hughes in Spiddal all spring to mind. Places where, even if there's nothing on that night, people will tell you where it is going on. And all places worth going to anyway. Unless you know your way around, better to avoid Dublin except for the obligatory look at the bullet holes in the post office.If you like cities, Cork is much more user friendly to the outsider, and allows access to West Cork and Kerry.
    Doolin's a funny one: I've never been there, having heard it heavily criticised as a waste of space by people whose opinions I respect, but equally well you meet loads of people who say it is the greatest place this side of heaven. One day I'm just going to have to go and take a look.
   Avoid Killarney, there is a lot of tourist music but it's pretty grim. I've played gigs there, but I wouldnt visit for pleasure, though having said that the countryside around is truly fabulous.