Of course even if the default meaning of session is an instrumental one (I don't subscribe to the late Sir Thomas Beeching's snide categorisation of Musicians and Singers) then perhaps you still need further description. The English Country Danceband Music session at Southwark would not suit many Irish Style musicians at all. Neither would one which specialised in Playford tunes or one I know at Eastwick which is mainly Bluegrass. I hope that the word does not come to mean only Irish music as many of our local ones seem to.