I can't talk for other sorts of players, but I'm pretty sure that the main motivation in Irish session music is sharing. If it's not, the session doesn't work. But sharing withe whom is the question. I think the answer is with those qualified to understand. They will include other musicians (sometimes not all...) and those of the boozing crowd who know what they are listening to (usually about 15% in a really good session).

I suspect this is more widespread than you think. For example, Zorba or Nureyev may have been great dancers, but it's totally lost on me. I can tell it's athletic, skilful and only a few people can do it, but beyond that I couldn't tell you if it's good dancing or bad dancing, and what is being communicated passes me by. I expect other people feel the same about session players.

Songs are a bit different- most people consider themselves qualified to understand them if they are in their own language. But even here, I think you fool yourself if you think others necessarily receive what you transmit.

With all music genres, a lot of the appreciation seems to come from the sense of a shared experience- your appreciation of the music identifies you with the environment (social, cultural, historical) in which that music thrives. Whether opera or ballet goers, blues fans, morris dancers or nightclub denizens, the music identifies a perceived lifestyle (whose details may not be the same for all participants note) with which you wish to be associated.