Very interesting. I was also just thinking about this and considering posting a similar question. Michael Robinson's discussion is here IRISH SESSIONS. A couple of comments from his discussion follow:

Hammy Hamilton, a well-known Irish flute maker, comments about the session and non-solo playing: "I've been working in this area for some time and I believe that 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 1950s! The earliest date that I can establish for a pub session is in the late 1930s and I think this would have been very unusual at the time."

Margaret Steiner at Indiana University also comments: "Traditionally, playing was a solo activity, around the hearth, and perhaps taking place in conjunction with other expressive arts - singing, storytelling, etc., among neighbors who gathered together at a céilí house. The céilí band, and later the session, were recent innovations. I can tell you about my experience in Newtownbutler, Co. Fermanagh. In 1978 virtually no instrumental music was heard in the pubs, although sometimes someone would get out his fiddle, or maybe there would be a fiddle and an accordion, but this was generally at Mrs. Connolly's, a well-known céilí house - i.e., a home where folk would spontaneously gather to socialize and sometimes to play music or sing or tell stories. When I returned to Newtownbutler, in 1992, sessions had made their appearance. For most people it was background music. The session music, brought in through radio and records, while pleasant to listen to, did not bear the same meaning for the community."

And Caoimhín MacAoidh writes (Between the Jigs & the Reels): "The decade of the 1960s saw a strengthening revival in traditional music. During the period the format changed radically. The primary venue changed from what had been the cottage to the new one of the pub. The music was now solely for listening purposes rather than dancing."

My question was, when did the pub session begin? It appears from the above that it was a recent development, and may have begun in America by Irish immigrants, rather than in Ireland. A related question, which I hesitate to post so as not to get into the endless PEL controversy, is when did the pub session begin in England?

Cheers, S. in Seattle