I'm sure that the term is derived from the American term "jam session" used by jazz and blues musicians. Within those genres you'll often hear the "session" dropped and it will just be called a "jam". Why Irish musicians decided to keep "session" and drop "jam" is the unknown. Perhaps because "jam" implies an emphasis on improvization and exploration whereas Irish sessions lean more toward tradition? It's not really a "jam" session if everybody is playing tunes they already know and they're all playing them the pretty much the same way.

Bruce