Play it in any key you like! I play it in C minor (3 flats), though as Bonnie points out, it’s in the Dorian mode. The only thing to consider is whether or not you’ll want to play it with other musicians, who will be cursing you if you’ve chosen a difficult key for them. (Guitarists and fiddlers hate C minor, and whistle players have no chance!)
I’ve never seen sheet music for it (aside from in the transcription from Alan Stivell’s ‘Renaissance of the Celtic Harp’, which is quite hard to get hold of these days). But it’s not too difficult to work out the tune for yourself, and then play around with accompaniments and ornamentation. It’s one of those pieces that I love to improvise around, you can do some quite bold things with the left hand harmonies and it sounds great 