Behan's own words: "Written in honor of Feargal O'Hanlan, a 17-year-old farm laborer from Ballybay, County Monaghan, who was killed during the attack on Roslea Barracks, in January, 1955 [sic]. The words and music to this song are my own composition" and "I wrote the song (words and music) on the 1st January, 1957".
Actually, the tune seems to be borrowed from an old folk song of unkwown origins.