Marje - As has been said, it's a trad Irish site primarily (which is the music I love) but much of the information is relevant, especially to the original poster who was asking about playing in sessions. I know the session gets some stick, but in my opinion it's true to the music and offers some excellent insight into playing Irish traditional music.

Perhaps there are English music sites that offer similar insight into accompaniment for English tunes (although I suspect there is some crossover)? I'd certainly be interested in seeing it.