My great grandfather was Patrick Montgomery and apparently he died in the last third of the 19th century.His son was known as William Hugh McGivern in his lifetime, and died in the 1920's. A tenant farmer he paid rents to the Downshire Family, and in their records he is known as William Hugh Montgomery of Ballykinler (near Clough). This is "anecdotal" but reasonably plauble since I've had it confirmed by my own father, and by three of his sisters and two of his brothers. (At various times one or other had done some "research" with little results beyond this). Also, the Ulster Historical Society noted the same information. (They have access to the Downshire Land records) Unfortunately, there are no survivng documents.
An intriguing puzzle which over the years has eluded me !Any information you had would be keenly appreciated.