I have been in one or two Irish sessions listening to fiddle players, and no one ever says what key they are in or anything, but people seem to be able to follow it (is that because they are all in D or something?)

If it is in a major key - if its not in D it will be in G and if not it will be in A.

If it is in a minor key - if it is not Am it will be in Em.

But there is a little bit more to it than this.