I'm impressed that the Goleen has a website, but the link didn't work for me. If you like unaccompanied traditional singing, this is one you shouldn't miss.

If you want to hear tunes here are a few starting points :

The Chancery Inn (new name over the door maybe O Reilly's) on the Quays (banks of the Liffey) near the Four Courts on Thursday night.

Hughes Pub Chancery Street, (very near to the Chancery Inn) any night. There's usually a session in the front part of the Pub (the snug) if you want to join in and another in the back which has excellent musicians, but not so friendly for joining in usually.

The Cobblestone, Smithfield usually a session or two at ground floor level and a venue upstairs for pay in gigs (usually folk or at least acoustic stuff). Sunday lunch-time session in the bar (1-4pm ish) is usually good here.

During the day (4-7?) there are sessions in a few pubs in the Temple Bar area, including in The Oliver StJohn Gogarty. These tend to be put on mainly for tourists, but there are some good players in some of these sessions. Have a walk around Temple Bar and try a few different ones to see if you can find something you particularly like before settling in to one of them.

There are sessions in Temple Bar at night as well, but there's usually better to be found elsewhere. On the other hand for someone celebrating a 21st Temple Bar is a part of town where they'll meet plenty of people their own age.

I don't get into the City Centre too much these days, so there may be a lot going on that I haven't heard about - go to some of the places I've mentioned and try talking to the Musicians or some of the people actually listening to the session (the bar staff probably won't have much of a clue).