The only evidence I have that a Samuel McDonald served in the 91st Pennsylvania is that the pension index by regiment (1) includes him in the 91st. The regiment listed on the card is light and hard to read, but could be '91'. However, since neither Bates nor the Pennsylvania Archives Civil War Veterans' Card Files have a Samuel McDonald in the 91st, I suspect this is an error. Perhaps the pension index card is really the Samuel McDonald who served in the 97th Pennsylvania Infantry (from 3 March to 28 August 1865), according to the Pennsylvania Archives cards.
According to the pension index (1), he served in company I [?] of the 91st Pennsylvania Infantry, and also in "E 6 US Ca[valry?]" (1).
On 11 August 1866, his father applied unsuccessfully for a pension (1).
NOT IN 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.
NOT IN Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 26 December 2004
1 pension index, by regiment (Samuel McDonald)