He was 30 years old when he was enlisted (3). (If I have located the correct death certificate, he was 27, and born in Ireland (5).)
He was enlisted and mustered into service on 11 September 1861 (1, 3). He was enlisted at Philadelphia, Pennsylvania (3). He was mustered in as a private, at Philadelphia, Pennsylvania (3). He served as a private in company D (1).
He died of "cramp colic", at Philadelphia, Pennsylvania, on 9 or 20 October 1861 (1 [20 Oct], 2 [9 Oct], 3 [20 Oct]). If I have located the correct death certificate, he died on 3 October 1861, of natural causes, and was buried on 5 October 1861, in Odd Fellows' Cemetery.
On 22 May 1863, his mother, Mary Irvin, applied successfully for a pension (4, 6).
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster) (David Irvine)
2 company D, register of deaths (David Irvine)
3 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 9 August 2004 (David Irvine)
4 pension index, by name (David Irvin)
5 death certificate, Philadelphia, Pennsylvania, 3 October 1861 (David Erwin)
6 pension index, by regiment, 91st PA Infantry, company D (David Irvin)
[I did not find an entry for him in the index on <www.ancestry.com> to the 1890 veteran's census (searched May 2007)]