The Mother of all Sessions.
Sessions start at 10 pm and make sure you are there by then if you want a seat. Go to the third room back for the sessions. The forth (last room) is more for Irish set music. I would not suggest jumping into a session here if you where a beginner. Although I have seen it done, and the crowd gracious, it seems a little inappropriate unless discussed in advance. I have seen everything from little 13 year old girls (playing fiddle) to old masters to Matt Molloy himself playing here.
Re: Matt Molloy’s Bar
True, don’t jump in if you’re a beginner. BUT you may be welcomed to join anyhow. Anecdote: I visited in 2013 (while Matt was away on travels). I set my mandolin case against the wall, but after a few tunes the session leader asked whose mandolin that was. Mine, said I. Get it out, said he. No, said I: I’m not good enough. But shortly after, having recognised my American accent, he started playing "The Tennessee Waltz", looked at me and nodded at my mando several times. OK, good excuse to join. After that, I joined in any tunes I knew. - And another attendee had heard me at another Irish pub session the evening before, with my friend Okinawan professional folkie Eriko Uehara, who felt a stylistic link between sean nós and some Okinawan styles and so moved to Westport. Eriko was regularly asked to sing Okinawan songs, so I joined her in them and mainland Japanese songs. (I’m a Japan/Okinawa folk specialist.) That tourist gal asked me to sing the oxherd song I’d done the previous evening ("Nambu Ushioi Uta"), so I did, and the pub musicians were very welcoming. In short (that was long!), don’t jump in at first, but be ready and willing if you sense the opportunity.