You don’t say what denomination the Duff family were, but if they were Church of Ireland then the good news is that the local baptism records go back to 1767 and marriages to 1778. They have also been indexed, and there’s a copy in PRONI (the public record office) in Belfast. If they were of another denomination then sadly the records don’t go back far enough.
I looked in Griffiths Valuation for the mid 1800s but there were no Duff households listed in Kilskeery. The 1901 census has 108 people of that name, a mix of RC, Presbyterians and Church of Ireland. I noticed one Alexander Duff in Drumaslaghy:
That’s quite a bit east of Kilskeery and may just be a coincidence. They were Presbyterian (indicating they were of Scottish origins and probably settled in Ireland in the 1600s).
There were no Duffs listed in the tithe applotment records of the parish in 1826. So the name must be fairly rare in the area: