As shown in the original manuscript, the original key is A: very nice if played in the lower register first time round then up into the higher octave for second time round.
However, to suit whistle players and those who can't go so low, it is often played in D. I have a 3-part arrangement in D, with chords, for 2 treble instruments + cello: if you want it, just send me a pm with your email address