"Danny Boy" was an Irish song from the early 20th century.
In the Fair Haven holoprogram, The Doctor promised to sing "Danny Boy" at Sullivan's in 2376. (VOY: "Fair Haven")
From the Star Trek Encyclopedia (4th ed., vol. 1, p. 180), "Earth folk ballad composed in the 1600s, usually sung with lyrics written by Frederick Edward Weatherly in 1913." The authors described the song as the "telling of a parent's wish to be remembered by a child." Lastly, it is said that The Doctor kept his promise.