Bates (1) has entries for Patrick Cahill (F, mustered in 2 Oct 61) and Patrick Cohill (D, mustered in 3 Oct 61). Since Patrick Cohill was not on company D's muster-out roll, I have assumed they are the same person.
He was 19 years old when he enlisted (5, 8). He was born in Ireland (8).
He was a laborer when he was enlisted (8).
He was 5 feet 6 inches tall, and had a light complexion, blue eyes, and light hair (8).
He was enlisted and mustered into service on 2 October 1861 or 3 October 1861 (1, 5 [3 Oct], 8 [2 Oct]). He was enlisted for three years, by Lieutenant Pierce, at Philadelphia, Pennsylvania (5, 8). He was mustered in as a private, at Philadelphia, Pennsylvania (5). He served as a private in company D (1).
He was transferred to company F on 31 October 1861 (4).
He was killed on 18 June 1864 at Petersburg, Virginia (1, 2, 6, 7, 8).
On 17 October 1864, his mother, Ann Cahill, applied successfully for a pension (3, 9).
On 28 September 1868, his father, Bryan Cahill, applied successfully for a pension (3, 9).
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) (Patrick Cahill)
2 Thomas Walter. 'Personal recollections and experiences of an obscure soldier'. Grand Army Scout and Soldiers' Mail volume 3, number 48, page 2.
3 pension index, by name (Patrick Cahill)
4 company D, register of men transferred, entry 3 (Patrick Cohile)
5 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 8 August 2004 (Patrick Cohill)
6 consolidated morning report, 91st Pennsylvania, 29 June 1864 (Private Cahill)
7 company F register of deaths (Patrick Cahill)
8 company F, descriptive roll, #24 (Patrick Cahill)
9 pension index, by regiment, 91st PA Infantry, company F (Patrick Cahill)
[I did not find an entry for him in the index on <www.ancestry.com> to the 1890 veteran's census (searched April 2006)]