Campbell is a pretty common name in Co. Donegal. There’s 1180 people of that name in the county in the 1901 census. I think the original Campbells there were Scots from the western islands of Scotland who were invited to come to Ireland as mercenaries (in the 1300s or 1400s) and were given land by local chiefs in return for their fighting skills. Such mercenaries were known as Gallowglass.
The problem you face is that the births, deaths and marriages are all before the start of statutory registration in Ireland (1864). Normally you would be able to rely on church records to get back a bit further but in Donegal none of the parishes have any records much before 1850. RC parishes generally don’t keep burial records either so that won’t help with tracing the graves.
Your best bet might possibly by DNA comparison, because I think you’ll struggle to find any paper records.