Not being that knowledgeable about Irish music, I did a web search and found this site which had info and etiquette. In reading it, I started to feel a little unwelcome, particularly as a guitar player. It starts out friendly enough:
Seisiúns are usually open to anyone that wants to join in provided they already know how to play Irish traditional music.
but then goes on:
...you might want to observe it first
...if you don't know the tunes that are being played you wouldn't want to try to play them…
...Trying to learn the tunes at the same time others are trying to play them can be very annoying…
...be careful when joining a session not to start tunes without being asked
…not disturb the way that the music flows.
I'm sure that it's annoying to have people turn up that have no idea what they are doing and bang away, but at the same time, it seemed a little rough. Or maybe I'm too insecure and sensitive.