Irish for a
session - a gathering of musicians (and
punters) to enjoy some
Irish traditional music using implements such as
flutes,
fiddles,
uillean pipes,
tin whistles and (possibly one and no more)
bodhrans.
The seisiún has its own set of rules. On the surface it appears to be a chaotic gathering, but under that there is a good bit of etiquette that you are better off knowing about and adhering to.
The best first thing to do when you come across a session is to sit down and listen, even if you are really eager to get going on playing yourself. What are they playing? What kind of style is dominating the seisiún? What is the level of the players? Only if you feel that you can hold your own and blend in well should you ask to sit in.
Once in the circle of musicians, remember that you are a guest. Jump in when the tunes you know are played, but always listen more than you play.
All rules aside - good craic (or crack) is always the desirable end product of a seisiún.
Pick up Ciaran Carson's "Last Night's Fun" for a great read on the dynamics of the seisiún and the special time that flows with the music.