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.
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).
Some of the professionals were more generous, however, and some schools of playing can be traced back to particular founders.
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.
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 labor, 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.