From the National Archives and Records Administration:

"AGO records also reveal that on August 3, 1862, a nineteen-year-old Irish immigrant named Albert D. J. Cashier, described as having a light complexion, blue eyes, and auburn hair, enlisted in the Ninety-fifth Illinois Infantry. Cashier served steadily until August 17, 1865, when the regiment was mustered out of the Federal army. Cashier participated in approximately forty battles and skirmishes in those long, hard four years. After the war, Cashier worked as a laborer, eventually drew a pension, and finally went to live in the Quincy, Illinois, Soldiers' Home. In 1913 a surgeon at the home discovered that Albert D. J. Cashier was a women. A public disclosure of the finding touched off a storm of slensational newspaper stories, for Cashier had lived her entire adult life as a man. None of Cashier's former comrades-in-arms ever suspected that he was a she. Apparently, neither did the commandant at the Soldier's Home. She died October 11, 1914, in an insane asylum."

For more information on Women Soldiers of the Civil War, see the NARA website linked above.