When did Weatherly write the song, and when was it set to Londonderry Air? Dated copies among many songs by Fred E. Weatherly in the Levy sheet music collection range from 1882 to 1916, but "Danny Boy" and "Elle Dear" aren't there. Fuld, 'The Book of World Famous Music', 1966, says of "Londonderry Air": "Many lyrics have been set to this music, perhaps the best known being Danny Boy in 1913 with words by Fred. E. Weatherly".

Was that when "Danny Boy" was written, or when it was set to "Londonderry Air"? I suspect the latter.