Fold3's index lists him as serving in the 91st, but the National Archives found no Peter Donelly in the 91st, but a Peter Donnelly who served as a private (enlisted on 8 August 1863, born 1831 Scotland) in company B of the 90th Pennsylvania Infantry. And when I rechecked the index card, the second digit of the regiment does seem to be a very thin '0', and not a sloppy '1'.
1 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (Peter Donelly)