Well, Kenneth MacLeod and Marjory Kennedy-Fraser were the editors of Songs of the Hebrides, which first appeared in (I think) 1909.  It was a collection of Gaelic song harmonised according to the conventional drawing-room tastes of the day, with piano accompaniments.  Many of the songs were given with English language "singing" versions, which were not necessarily translations from Gaelic, and a number were composed by the editors themselves; usually this is made clear.  Probably this song is taken from that collection, which, though long out of print, is not difficult to find through public libraries.  It seems on the face of it to be unrelated to another song of the same name,  Mo Mhaire

Malcolm