Margaret Barry learned the song from a John McCormack record, it appears, and the introduction of "dead love" seems to derive from him, possibly as the result of an accidental misreading of sheet music. Certainly it doesn't appear in Colum's verses, or in any known traditional forms of the song on which he based it. The "mystery" is of recent origin. Those aspects of the song have been discussed here in the past at some length, of course.