I was in the Tass last night but I think it was too early for any music. I've an idea it doesn't start until 8pm, possibly 9pm.

I must say, though, I've never thought of the Friday night thing as a session per se, because it's only ever the same crowd who play and sing, and in many visits there, I've never seen anyone asked to join in or give a song. Not that it matters; it's an excellent way to spend a Friday evening.