JohnIK, I do not think "mythical true pitch tuning" is the main point. The rule "never play two pieces in the same key consecutively" applies to all music programmes including those of classical pianists, who always play in equal temperament.

Marje, I do not organise such events. My idea was: if there is a conflict between multi-keyers and everybody-play-alongers, separete sessions may stop them complaining. Besides, I now live in Germany and France, where folks have similar problems (with different preferred keys), and generally envy the British folk scene.

Jack Sprocket, you are quoting me incompletely. In fact Irish solo fiddling, like most fiddling traditions, uses empty strings for special effects, but still requires a very good command over the left pinkie. In contrast, the typical first position players Will had in mind tend to hide away their little finger. If they have a physical defect, they are excused. For example, the finger may have been cut off by kidnappers, when understandably nobody payed the ransom.