I started the discussion with the following ideas. Several people responded, pro and con. However, I'm only going to include the responses of people who agreed with me, because those who disagreed with me are obviously wrong. All the texts have been slightly edited.
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. 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.