The ever wonderful Jane Griffiths wrote this out for me from Planxty’s playing and since that’s the version we know and love, I think you’ll enjoy playing it. However, if you know a slightly different version, I’m sure it will make a lovely harmony part.
On thesession.org there are many variations on this tune (and a many other names too), but actually although it is mostly thought of as being Irish, largely for being played by Planxty, in the 18th century it was reported as being played in several places in the British Isles. Apparently, Merrily is recorded in
David Rutherford, Collection of Country Dances, London, 1750-60 (ish)
Robert Bremner, Collection of Scots Reels and Country Dances, 1768 (possibly)
And it has been claimed that the tune was put to words as an electioneering song in Newcastle in 1777 (I can find the words but no reference to confirm the tune!).
Download the pdf here.