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.
The objective in a session is not to provide music for an audience of passive listeners; although the punters (non-playing attendees) often come for the express purpose of listening, the music is most of all for the musicians themselves. The session is an experience that is shared, not a performance that is bought and sold.
Learn how to play in an Irish session here: http://celticmusicacademy.com
The sessions are a key aspect of traditional music; some say it is the main sphere in which the music is formulated and innovated. Further, the sessions enable less advanced musicians to practice in a group.
Socially, sessions have often been compared to an evening of playing card games, where the conversation and camaraderie are an essential component. In many rural communities in Ireland, sessions are an integral part of community life.
Typically, the first tune is followed by another two or three tunes in a set. The art of putting together a set is hard to put into words, but the tunes must flow from one to another in terms of key and melodic structure, without being so similar as to all sound the same. The tunes of a set will usually all be of the same sort, i.e. all jigs or all reels, although on rare occasions and amongst a more skilled group of players a complementary tune of a different sort will be included, such as a slip jig amongst the jigs. Although bands sometimes arrange sets of reels and jigs together, this is uncommon in an Irish session context.
Some sets are specific to a locale, or even to a single session, whilst others, like the "Coleman set" of reels ("The Tarbolton"/"The Longford Collector"/The Sailor's Bonnet"), represent longstanding combinations that have been played together for decades. Sets are sometimes thrown together ad hoc, which sometimes works brilliantly and sometimes fails on the spot.
After the set ends, someone will usually start another.
Sessions are usually held in public houses. A pub owner might have one or two musicians paid to come regularly in order for the session to have a base. Sunday afternoons and weekday nights (especially Tuesday and Wednesday) are common times for sessions to be scheduled, on the theory that these are the least likely times for dances and concerts to be held, and therefore the times that professional musicians will be most able to show.
Sessions can be held in homes or at various public places in addition to pubs; often at a festival sessions will be got together in the beer tent or in the vendor's booth of a music-loving craftsman or dealer. When a particularly large musical event "takes over" an entire village, spontaneous sessions may erupt on the street corners. Sessions may also take place occasionally at wakes. House sessions are not as common now as they were in the past. This can be seen in the book Peig by Peig Sayers. In the early stages of the book when Peig was young they often went to sessions at peoples houses in a practice called 'bothántiocht'.