The 1863 marriage certificate puts William Hemphill in the townland of Ballynally in the parish of Moville Lower. His father is named as James, a farmer.
There is 1 Hemphill family in Ballynally in 1901:
I don’t think this is your William’s father James. The James in the census married Sarah Peoples in 1875 and his father’s name was Thomas, a farmer. However he’s likely to be a relation. The family was Church of Ireland. The name Hemphill is common in Moville. The parish of Moville was split into Upper & Lower in about 1860. The early records are in Moville Upper. They start in 1804. You might want to check them for your William’s baptism. They are not on-line so far as I am aware but there is a copy in PRONI in Belfast.
Your question was whether William was born in Donegal. I don’t know the answer for certain but I think it highly likely. You would need to check the parish records to find out for certain.
Here’s a James Hemphill farming in Ballynally in the tithe applotment records for 1827:
Some general information on the parish: