Can't work it out meself (cloff-ear syndrome), are they playing BC or C#D boxes in the clip? https://www.youtube.com/watch?v=0iOK70Wc_Ro
Which key(s) are the tunes played in please?
Trick question! If you were just using your eyes, you would conclude that they were playing C#/D boxes, since all the D phrases in St. Anne's reel (originally Le reel de Ste-Anne [place, not person]) are on the push. Check the tuning however and you will find they are playing both tunes in C (St. Anne's and Humours of Tulla).
Answer: they are playing B/C boxes C#/D style (and the banjo must be tuned down a tone). Reverting to type, obviously!
All the brothers are obviously past their best at this stage, especially John, who wasn't long for this world when the clip was recorded. I think Joe has slowed down a lot, too. I remember seeing him play in a session in about 1982. It was stunning. Everyone stopped playing their instruments and just listened to Joe, who was on fire.