Thanks for all the help. You we appreciate, it is not a Welsh tune I am looking for(as in "tune form Wales").What we have is an old Cumbrian(Scottish borders) tune. And I think it is barely, dimly possible that it's title might just conceivably be a memory when this part of England(and some contiguous parts of Scotland) were inhabited by people who spoke the British language which is now called Welsh. But I must admit I havent got a shred of evidence to back this suggestion up, nor even any credible suggestion as to what that name might mean(though the godly Mungo is the sort of region I was thinking about.
    It's really just the sort of thing us romantic old folklorists would like to be true! A more rational(and very prosaic) explanaton, is that Gillespie wrote the tune, named it for a pal called Dew Mungo(a noted local manufacturerer of contraband hooch), and couldnt spell Dew.