He was born in 1836/37 (4 (24 in 1861), 6 (24 in 1861)). He was born in Paisely, Scotland (6).
When he enlisted, he was a moulder (6).
When he enlisted, he was 5 feet 7 inches tall, and had a light complexion, blue eyes, and light hair (6).
He enlisted and was mustered into service, at Philadelphia, Pennsylvania, on 23 September 1861 (1, 4). He was enlisted for three years, by Captain Smith (4, 6). He was mustered in as a private in company D (4 (co.D), 7-9 (co.F)).
He was transferred to company F on 31 October 1861 (3).
He was reduced to the ranks on 18 January 1861 [sic; presumably 18 January 1862] (6).
He deserted on 20 March 1862, from Carroll Barracks, Washington DC (1, 5, 6). He was a private, in company F (7-9).
Advance the colors identifies him as the Sweeney who was shot shortly after grabbing a fallen flag on 18 June 1864, but it seems more likely this was Samuel Sweeney; the only evidence they have is an article citing his name as 'Sweeney'.
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)
2 Advance the colors, p.314. (James C Sweeney, co.D)
3 company D, register of men transferred, entry 13 (James C Sweeney)
4 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 10 August 2004 (James C Sweeney)
5 company F, register of deserters, #15 (James Sweeney)
6 company F, descriptive roll, #6 (James Sweeny)
7 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (James C Sweeney)
8 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (James H Sweeney)
9 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (James Sweeny)
Bates (1) has two entries: James C Sweeney (D), not on muster-out roll, and James Sweeny (F), both mustered in on 23 September 1861. I have assumed they are the same person, and the fact that the index to compiled service records doesn't have a card for James C Sweeney in company D strongly supports that.