Probably of little help, but a couple of notes from the OED Supplement of 1987.
Jam- first appeared in print, with regard to musicians getting together and extemporizing, in 1923. Jam Session appeared in 1933.

In 1944, the term appeared in a British journal, "Theology." "This contemporary jam session gives enormous pleasure to the participants. But we [Church of England] have had little enough success in charming the ear of the nation to the extent of persuading it to come and join the band." From this statement, I would guess that "jam session" was a well-established term in Britain by that date. By 1967, a Spanish dancer was speaking of a "flamenco jam session."
It has come to mean any get-together where people throw around ideas; it is used for company meetings in which people actively participate.

I am sure that all musicians with even marginal interest in jazz would have picked up the term very early, and it rapidly would become used with reference to any group getting together to play, improvisational or not, Irish or whatever.