“…a gathering of Irish traditional musicians for the purpose of celebrating their common interest in the music by playing it together in a relaxed, informal setting, while in the process generally beefing up the mystical cultural mantra that hums along uninterruptedly beneath all manifestations of Irishness worldwide.”
The general session scheme is that someone starts a tune, and those who know it join in. Good session etiquette requires not playing if one does not know the tune, and waiting until a tune one knows comes along. In an “open” session, anyone who is able to play Irish music is welcome.
Most often there are more-or-less recognized session leaders; sometimes there are no leaders. At times a song will be sung or a slow air played by a single musician between sets.