Why was Danny boy played at funerals?

Whilst certainly not an official hymn, the tune Danny Boy, which is sung to the tune of Londonderry Air, is sometimes thought of being liturgical in composition. As early as the late 1920's the tune was found to be considered as an unofficial anthem, extolling the love of Ireland and the nucleus of the family. It is a song offered as a parent singing to a son. This was particularly considered during the great Irish diaspora (migration) to all points around the world for either war or migration. It's interesting to note that the song was written by an Englishman, Frederick Weatherly, in 1910. Even he considered his song a fitting tribute for a funeral.