An excellent description of sessions, and Irish music in general, is to be found in World Music: The Rough Guide, published by The Rough Guides, Ltd. in 1994 (this book is incredible - get it if you can find it):

THE SESSION: MUSIC AND CRACK

The session is the life-blood of traditional music, and with the session goes the associated notion of crack. Crack is hard to define and impossible to plan for, and when it happens is obscurely described as being "ninety" or "mighty", as in "there was a mighty session last night in X's and the crack was only ninety". Music, conversation, drink and people combine in mysterious fashion to produce good, or even great, crack.

As a rule sessions take place in pubs, the temples of Irish traditional music culture. Many pubs and bars hold pay-on-entrance sessions or gigs (after which a "real" session might happen) but they also cater for musicians who need a place to meet and play informally. In this case the pub owner is usually into the music and not just out to make a fast punt. Under this arrangement the musicians are not paid, but neither are they under any obligation to play, or even to turn up. It's possible to arrive at a pub known for its sessions only to find that on this particular night no one is in playing mood. The venues of sessions are as changeable as their personnel, and situations can change overnight. A change of ownership, a row, or too many crowds can force the musicians out to other meeting places. Nevertheless, summertime is a good time for sessions, particularly in the west of Ireland, and a few inquiries locally will usually yield the necessary intelligence.

At first sight sessions may seem to be rambling, disorganized affairs, but they have an underlying order and etiquette. Musicians generally commandeer a corner of the pub which is then sacred to them. They also reserve the right to invite selected nonplaying friends to join them there. The session is not open to all comers, although it might look that way, and it's not done simply to join in with no form of introduction. More than one session has been abruptly terminated in full flight by the insensitive or inebriated ignoramus insisting on singing "Danny Boy" or banging away inexpertly on the bodhrán (Irish frame drum) in the mistaken belief that his or her attentions are welcome. The newcomer will wait to be asked to play, and may well refuse if they consider the other musicians to be of a lower standard than themselves.

Good traditional sessions can feature group playing, solo playing, singing in Irish and English or any combination of these: it all depends on who's in the company and where their musical bias lies. Singers may gang up and keep the musicians from playing or vice versa. The all-inclusive session often occurs at festivals during the summer when large numbers of musicians congregate in one place. These can be unforgettable occasions when it seems the music just couldn't get any better and all its treasures are on display.

Basically, the "rules" are:

• 1. Don't join in without asking, or without being invited to play (if you're at an unfamiliar session).
• 2. Don't play if you don't know the tune (it's usually OK to play quietly off to the side if you're trying to learn the tune).
• 3. Don't hog the session; that is, don't try to start off, or play between, every set of tunes. The session is not the excuse to play; rather, the music is the excuse for the musicians to meet and have a good time. It's not necessary to have music going constantly.
• 4. Try to be aware of the "crack level", and try to foster it in your choice of tunes. Don't sing or play something you know no one is going to be interested in.
• 5. Don't expect musicians at the session to teach you how to play (though you will often find those who will be happy to).
• 6. LISTEN to the other musicians. Don't get off-time or out-of-tune with them. If there seems to be a leader in a set of tunes, follow him or her.
• 7. Enjoy yourself.