Danny Boy is one of over 100 songs composed to the same tune. The author was an English lawyer, Frederic Edward Weatherly (1848-1929), who was also a songwriter and radio entertainer. In 1910 he wrote the words and music for an unsuccessful song he called Danny Boy. In 1912 his sister-in-law in America sent him a tune called the Londonderry Air, which he had never heard before. He immediately noticed that the melody was perfectly fitted to his Danny Boy lyrics, and published a revised version of the song in 1913. As far as is known, Weatherly never set foot in Ireland.
The tune was not collected by Lady Londonderry.. as stated above... It was collected from a lady in Londonderry in I think 1863 .. which is hardly the same thing.