Answering Jim B first: and yes, I know this thread was supposed to be mainly about sessions, but the event I was talking about last weekend was a workshop rather than a session, so sheet music was used to allow for a larger number of tunes to be got through in the course of a day. This included standard notation, mandolin tab and whistle diagrams for the appropriate instruments. And with over 30 of us in the room, it’s better that the chord players all play the same chords. When I used to go to sessions I was mainly playing by ear and leaving the dots at home. But, sadly, I am losing my hearing, along with my sense of pitch, so playing by ear is no longer at all easy for me. So it’s back to the dots.
And yes, it does seem that a lot of piano accordion players do favour using sheet music, though there are also plenty who manage without. And no, I don’t think it is an increasing trend for semitone melodeon players to use the dots: as you know, our choice of chords is limited, but some fit better than others!

And now answering Lang Johnnie Mor: there ARE branches of Comhaltas Ceoltoiri Eireann in Scotland, notably the long-established (35 years) Glasgow branch which has spawned many brilliant players of Irish music. Much more recently, only this September, a new branch has started in Edinburgh: so far they have had a couple of instrumental sessions and another meeting focussing on song.