Alexander Milroy (1846 - 1881) also joined the Union Army (in addition to his father Robert and brother William) - he signed up with the 20th NYS Militia (80th NYSV). He apparently gave his age as 18 when he signed up in October of 1861. According to the family Bible transcription above, he would really have been only 15 or 16 at the time. At some point during his service, he was promoted to Corporal. His description is given as "5 feet 3 1/2 inches tall, light complexion, dark eyes, light hair." Army records show him as being discharged in 1865. On the Sons of Union Veterans Civil War grave database, there is an entry for MILROY, A. from the 80th NYSV, Company A. He is buried in Oak Hill Cemetery in San Jose, California. The Sons of Union Veterans database gives the death date as December 17, 1881. Headstones Provided for Deceased Union Civil War Veterans, 1879-1903, found on Ancestry.com, gives his death date as December 29, 1880. This is confirmed by the Register of Deaths for Santa Clara County in California.
A New York town clerk's record has the following note for Alex: "Deserted, was retaken, and is Still in Service". Alex apparently stuck with the army after he was retaken. The register is dated "ca. 1865-1867". Other records show him being discharged in 1865, so he doesn't appear to have stayed with the army very long after the war. Still not sure how much action he saw in service, if any. I recently found "New York, Registers of Officers and Enlisted Men Mustered into Federal Service, 1861-1865" on Ancestry.com, which further states that Alexander was punished for his desertion by being held in the service for an extra year. It also states that "In 80th Vol. Helped to plant the first flag in Petersburgh, Va."