As a hymn-tune, it's usually called "Bunessan", to be found in hymnbooks to the words of a carol, "Child in the Manger". My hymnbook says the "Gaelic Melody" is "arranged by David Evans, 1874-1948". The words are a translation by Lachlan MacBean (1853-1931) of a Gaelic hymn by Mary MacDonald (1817-1890 or so), to be found in MacBean's "Songs and Hymns of the Scottish Highlands" (1888), 24, where we're told M.MacD. was from Mull; Gaelic designation Mairi Dhughallach, bean Neil Dhomhnullaich ann an Ard Tunna. Her hymn is called (and begins) "Leanabh an aigh".