In one of our sessions we don't seem to have much of a problem with tuning - everyone sorts it out by finding a note and tuning to it. On the other hand, we have another session where despite the fiddles being initially in tune, the players always tweak them up, never down, so during the course of the night all the fiddles go sharp and then other instruments try to follow, until a box joins in then there's lots of accusing glances and more tweaking. But mentioning instruments out of tune in a session can set off some touchy people. We usually just say, "I think I've gone out of tune, anyone have an A?" then most folks get the idea and retune.

By the way, there's a wonderful collection of Irish tunes collected by Dr George Petrie a couple of hundred years ago. He made the mistake of carrying around a tuning fork and then writing down lots of the tunes exactly in the key they were played... so we have lots of tunes in Gb, Ab, Db, Eb, Bb minor, F minor and so on. The music is peppered with accidentals and the easiest way to figure out what's going on is just to rewrite them out in a sensible nearby key.