I suspect he is the same person as John
He was 23 years old when he was enlisted (4, 5, 9, 10 [25 in 1863]). He was born in New York, New York (5, 9, 10).
He was a moulder (4, 5, 10). He was living in Philadelphia, Pennsylvania, when he was enlisted (4, 5).
He was 5 feet 6 inches tall, and had a light complexion, blue eyes, and light hair (4, 5, 9, 10).
He was enlisted and mustered into service on 23 September 1861 (1, 4, 5, 9). He was enlisted for three years, by Captain Smith, at Philadelphia, Pennsylvania (4, 5, 9). He was mustered in as a private, at Philadelphia, Pennsylvania (4, 5). He initially served as a private, perhaps initially in company A (1).
He was promoted to corporal, probably on 1 January 1863, though perhaps on 1 June 1863 (1, 5, 8 [1 Jan], 9 [1 Jan]).
He reenlisted as a veteran volunteer on 31 December 1863 (1, 4, 5, 10 [30 Dec]). He was reenlisted for three years, at Warrenton Junction, Virginia, by Lieutenant W H Carpenter (4, 5, 10).
He was transfered to company F on 16 February 1864 (3). He may have been mistakenly transferred to company A when he reenlisted (see fact of the week #102).
He deserted from Philadelphia, Pennsylvania, on 28 April 1864 (1, 2?, 5, 6, 7, 9, 10).
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)
[Bates lists John Walsh in co A as not accounted for at muster out. His source obviously missed special order 4, HQ 91st PA, 16 Feb 64, which transfers Walsh from co.A to co.F.
2 letter, Sinex to Fowler, 28 April 1864
3 special order 4, HQ 91st PA, 16 February 1864
4 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 18 August 2004 (John Walsh [co.A])
5 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 18 August 2004 (John Walsh [co.E])
6 consolidated morning report, 91st Pennsylvania, 28 April 1864 (Corpl Walsh)
7 company F, register of deserters, #28 (John Walsh)
8 company F, list of non-commissioned officers (John Walsh)
9 company F, descriptive roll, #38 (John Walsh)
10 company F, second descriptive roll, #16 (John Walsh)
[I did not find an entry for him in the index on <www.ancestry.com> to the 1890 veterans' census (searched November 2005)