it is, oddly, notated mistakenly in Gmajor in the Northumbrian Minstrelsy

I have the facsimile from Llanerch, and that has an erratum squiffily printed above the score explaining that key signature is wrong. It must have been been inserted in a very early printing, so I doubt they left the mistake for very long.

I have all the Minstrelsy tunes in ABC on my site:

http://www.campin.me.uk/Music/BruceStokoe.abc