The list of non-commissioned officers (3) has his name as Frank P Donahue (see also 7).
Bates lists two Francis P Donahue's, one who enlisted in company B on 20 August 1861, and was not on the muster-out roll, and another who enlisted in company E on 20 August 1861, and mustered out with his company. I have assumed that they are the same person, who transferred from company B to company E, but that assumption may be wrong. Nothing in the company E descriptive book (4) suggests it is right, but it begins with his re-enlistment in 1864.
He was born in 1842/43 (4 (18 in 1861), 9 ). He was born in Philadelphia, Pennsylvania (4, 9).
When he enlisted, he was a vendor, and was living in Philadelphia (4, 9).
When he enlisted, he was 5 feet 6-3/4 inches tall, had a light complexion, blue eyes, and light hair (4, 9 [5'7-1/2"]).
He enlisted and was mustered into service as a private in company B on 20 August 1861, at Philadelphia, Pennsylvania (1, 9, 14, 16 (pvt, co. E)). At some point he transferred to company E (see previous paragraph).
He was detailed as an orderly at Convalescent Camp, Alexandria Virginia, by an order dated 1 Jan 1863, and was still there 13 June 1863 (2), and on 8 September 1863, when Lieutenant Colonel Sinex asked for his return (8).
He re-enlisted as a veteran volunteer (1), on 28 March 1864, at Warrenton Junction, Virginia (4, 9 [30 Mar 64]). He was enlisted by Lieutenant Brass, for three years (4). He was mustered into service by Captain Barnard (this may relate to his original enlistment) (4).
He was arrested 10 January 1865 at Philadelphia Pennsylvania (5, 7). He was tried by a court martial convened at Alexandria Virginia on 23 January 1865, for desertion (7, 15). He was found innocent of desertion but guilty of absence without leave (5, 7). On 4 February 1865, he was sentenced to lose all pay and allowances due him, and to make good the time he had missed (4, 5, 7).
He was not on the company B muster-out roll, but mustered out 10 July 1865 with company E (1, 9, 14). He was a corporal, with company E (16).
On 4 February 1880, he successfully applied for a pension (12, 14).
In 1890, he was living in Philadelphia, Pennsylvania (10). He may have been a laborer, living at 1208 South 11th Street (10, 11). If so, he had been wounded in an arm (10). However, neither of the census entries I have found that seem to refer to him match the other information I have about him closely.
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)
3 list of non-commissioned officers, company E
4 descriptive roll, company E, entry 30 (Francis P Donahue)
5 register of deserters, company E, entry 19
6 special order 55, headquarters, 91st Pennsylvania Infantry, 30 June 1864
7 general order 104, headquarters, Military Governor, Alexandria, Virginia, 4 February 1865
8 letter, Sinex to Marvin, 8 September 1863
9 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 6 May 2004 (Francis P Donahue)
10 1890 US census, veterans schedule, Pennsylvania, Philadelphia, Philadelphia, supervisor's district 1, enumeration district 477, page  (image 1875 on Ancestry), line 13 (Frank Donahue)
11 1890 Gopsill's Philadelphia directory (Frank P Donahue)
12 pension index, by name (Francis P Donahue)
13 'The Ninety-first Pennsylvania volunteers', Philadelphia Inquirer 8 June 1864 page 3 (Francis P Donahue)
14 pension index, by regiment, 91st PA Infantry, company E (Francis P Donahue)
15 National Archives Archival Research Catalog (accessed 24 July 2010) (Frank P Donohue)
16 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (Francis P Donahue)