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:
Some people are under the impression that Irish music sessions are a type of traditional event. However, authorities such as Breandan Breathnach and others agree that Irish music as played traditionally was a solo, unaccompanied musical form. Furthermore, the artistry of the music depends for a large extent on the variation and ornamentation of the basic tune by the performer - subtleties which are necessarily lost when there is more than one performer. In Cape Breton, which has probably the most conservative tradition in Gaelic music, it was unheard of until quite recently to have more than one fiddler playing at a time. To play while another person was playing would have been considered just as rude as talking while another person was talking. The only circumstance in which it was common to have more than one person playing at a time was at dances. The lack of affordable PA systems in the eighteenth and nineteenth centuries made it necessary to have multiple performers so that the music would be audible. From the reports of some of the early collectors, it appears that many professional musicians avoided performing in the presence of other musicians for fear that their tunes would be stolen. The stock of tunes in a given area may have been quite small and knowing a tune that others didn't could be a distinct advantage. Many of the old musicians were extremely jealous of each other and would carry their special tunes to the grave rather than teach them to anyone other than possibly a son or extremely well-loved pupil (with instructions not to perform them during the teacher's lifetime). While there were of course many talented amateur musicians, traditionally the best musicians were usually professional or at least semi-professional. However, being a professional musician in the early 19th century was a career rather similar to being a professional beggar. They often played for tips at cattle fairs, horse races, etc. A number of professional musicians in the old style kept going well into the 20th century. For example, Johnny Doherty and Padraig O'Keeffe made their livelihood from music without giving concerts until late in their lives, if at all (aside from being taped and played on the radio). The old harpers were almost all professionals, but they were usually maintained by the old aristocratic families. This form of patronage died out around the middle to late 18th century. In Scotland professional musicians adopted the modern style of giving concerts, going on tour, etc. around the middle 18th century, just as the old patronage system died out. The musician/beggar lifestyle existed as well - no doubt it depended on your class origins. 
The guess is that amateurs were much more likely to play in sessions than professionals, lacking the jealousy caused by having to depend on your store of tunes for your bread and butter, and lacking the artistry to perform elegant variations. Since such professional musicians as emigrated to the U.S. from Ireland in the 19th century tended to gravitate towards stage performance (since the opportunities for a traditional musical lifestyle and prejudices against lower-class performers appearing on stage were both absent), it may well be that the establishment of the session as the standard venue for the performance of Irish music was an American innovation. It is certain that the growth of sessions has changed the form of Irish music. The amount of variation of the tunes has decreased radically and the old descriptive pieces of music have almost totally died out. The lifestyle in which Irish music originated is almost totally gone and before we become too nostalgic about it we should remember that it was a life of hard physical labour, grinding poverty, poor health and early death. The fact that the music is changing is an indication that it is still alive and has not become a museum piece. This is not the first time that the music has changed in order to adapt to changing social structures, by any means. There is a strong connection between the improvement in social and economic conditions in Ireland at the end of the 19th century and the rise of amateur playing of traditional music. It seems that previously the vast majority of players were professional. Non-solo playing doesn't really appear until the early recordings of the 78 RPM period in the States. The session as we know it today is a much later development, in the majority of cases not being common until the 1950's! The earliest date that it can be established for a pub session is in the late 1930's and this would have been very unusual at the time.