After military experience in the Mexican Army in the early 1860s, he became active in the military affairs of the IrishRepublicanBrotherhood?, FenianBrotherhood?, and later ClanNaGael. Characterized as “an adventurer” by John Devoy, Millen was involved with the JubileePlot? of 1887. Apparently he was also on the British SecretService payroll at times.
Short, K.R.M. “Biographical Notes.” The Dynamite War: Irish-American Bombers in Victorian Britain. 261-66. __Atlantic Highlands, NJ: Humanities Press, 1979.