An ba shi izini a matsayin mataimakin mataimakin a cikin Rundunar Sojojin Amurka a ranar da ya kammala karatunsa. Irwin daga baya ya yi aiki tare da sojan doki a lokacin
Pancho Villa Expedition, wanda Brigadier Janar John J. Pershing ya umurce shi, a matsayin memba na 11th Cavalry Regiment a 1916 da kuma shekara mai zuwa.