Great instrument for traditional tunes. I've had the pleasure over the years of playing and recording with Barrie Marshall and Peadar Long, both great players.The huge range of a clarinet is very useful...youve got a soft bass to a fiddle or a wild high lead, all in the same instrument.
   Agree with Pied Piper, a C clarinet might be your best bet for playing in trad sessions, though it does mean you'll be missing the very useful low D note which is so handy on the Bflat clarinet(the C instrument only goes down to E).
    What I really like is the little Eflat clarinet, a fantstic instrument for outdoor procession bands, but probably a bit scary sitting next to one in a pub.