The advice above about fiddlers practising so that every key is the real key to the problem (sorry 'bout that) and it sounds like Mick's Natalie is a treasure.

The info about retuning the fiddle, however, can be helpful in some situations. A festival (now almost 45 years old) I go to at Christmas is located where the locals have traditionally played single row melodeons, in C, for most of the last hundred years. Concertinas fit in OK with the locals (some of whom play them and one of whom makes them) and guitarists are either equipped with capos or superior skills.

Fiddlers fit into a couple of categories. Those who've mastered their fingering use their onstruments as they are while others (some of whom could easily play in any key they want) 'take a holiday' and retune the fiddle so that their tunes in D fall out happily in C with no changes. Fine if the tunes in C are all 'originally' in D, but those 'originally' in G or A, or which modulate between keys, still require refingering.

I have noticed, at this festival, that some fiddlers take up other instruments if they want to play along with the C boxes. Or they just put their instruments down and get stuck into the dancing instead.

Cheers, Rowan