What is a session?
An Irish Session is a gathering of musicians playing traditional Irish music (occasionally including other Celtic genres such as Scottish, Brittany, Cape Breton, and French Canadian) on traditional Irish instruments. Traditional Irish music is made up of dance tunes such as reels, jigs, hornpipes, slides, and a few other miscellaneous forms including polkas, set dances, airs and songs (although songs are infrequent at the EOWS session). It is not so much a performance but a sharing of traditional tunes among the musicians and those who care to listen. The instruments might include fiddle, flute, accordion, uilleann pipes (pronounced ILL-in), concertina, tenor banjo, whistle (a.k.a. penny whistle, tin whistle), mandolin, bodhrán (pronounced BOW-ron), guitar, and sometimes piano. An Irish Session is not a "jam session".
If you are new to the session it is advised that you approach the session leader(s) and ask permission to sit in. Melody instruments should stick to the melody of the tune that is being played. There is of course, plenty of room for individual embellishments (ornamentations) within the framework of the tune. In almost all cases, it is really not acceptable to do improvisation on a melody instrument at an Irish Session. If you're not familiar with a tune being played at a Session and are trying to learn it, try to play very quietly, so as not to disrupt the flow of the tune. Disrupting the tune by playing “around” the melody (i.e. playing notes in the same key, but not the particular melody) is also known as “noodling”, which is generally annoying to those playing the actual melody. It is advised that you sit and listen so that you can become familiar with the tune. It is acceptable to record the tunes so that you can learn it on your own time.
The melody is the priority in a session, while the melody may be accompanied by rhythm on bouzouki, guitar, and/or bodhrán. However there should not be more than one of each rhythm instrument being played at one time, so as not to overwhelm or distract from the melody. The rhythm instruments are there to compliment the melody and to enhance the rhythm, which is also a key component of traditional Irish music, being that the tunes are usually dance music (reels, jigs, slip jigs, slides, hornpipes, polkas) where the dancers use their feet to beat out the rhythm of the dance.
Tunes are usually grouped into sets. Frequently these sets consist of particular tunes played in a particular order. There are sets that are traditionally grouped tunes, wherever you go in the world, while other sets are particular to an individual session. The tunes that are played in a session are usually verbally agreed upon before starting them so that there is some communication about what is being played next. It is not advisable to just launch into a tune without any communication. Paying attention to what the session leader(s) are communicating cuts down on chaos in the session. Tunes are usually played in a straightforward manner, which means that there is no “arrangement” which varies from the traditional tune, such as what a band might do to link tunes together with bridges during a performance.
If you have a tunable instrument (stringed, flute, most whistles) you should tune it so that you are not distracting from the melody. If you are playing with a non-instantly-tunable instrument such as an accordion, it is advisable that everyone tune to them, assuming they are reasonably close to being in tune.
The point of a Session is to have fun! If everyone abides by the usually unspoken etiquette guidelines, it leads to a more enjoyable time for all participants.
Further discussion on Irish Session Etiquette