The last note works most times, though there are exceptions - but you still have to listen to how the tune sounds, to be sure if it's minor or major or in some kind of mode.

If there's a guitar player who goes in for full chords and doesn't have a funny tuning, that's handy for spotting the key. Or, with some electronic tuners, you can spot the last note while you're up at the bar, with the idea of casually coming in with the right chord as if you had perfect pitch.

The basic rule though is, if you aren't certain, play softly and listen to yourself (stick your ear against the instrument) before you play loud enough for anyone else to hear.

D, G, occasionally A, plus a minor and e minor - that will get you most places in an Irish session. And C as well, if the tunes turn English. And then, of course, there are the times when the key changes between the tunes in a set. (When that happens inside tunes, that's not traditional, and you don't want to join in with that stuff, and they don't want you to either most likely.) There are rules and conventions here, but they vary.