A second and much smaller Ballyscullion House was built in 1840 for Admiral Sir Henry William Bruce, and is now owned by the Mulholland family.
A military camp was built on the estate in the Second World War by the 202nd Engineering Combat Battalion and occupied by the US 82nd Airborne Division before D-Day.
Downhill was damaged by fire in 1851, restored, fell into disrepair while occupied by the Royal Air Force in the Second World War, and now lies in ruins.
The design may have been based on John Plaw's Belle Isle, Windermere, or Palladio's Villa Capra, both themselves inspired by the Pantheon in Rome.
The northern façade resembles St Peter's Basilica in Rome, with a central domed rotunda flanked by curved wings, and a large pavilion at each end, forming a front measured 350 feet (110 m) in length.
A double corkscrew staircase lit from above - possibly inspired by the famous staircase at the Château de Chambord - led up to a circular drawing room on the first floor, with views across to the Antrim hills to the east, the Mountains of Mourne to the south and the Sperrins to the west.
Alexander donated the columns to be incorporated in the rebuilding of St George's Church, Belfast, where they form its portico.
In the Second World War, the estate, renamed Camp Ballyscullion, was prepared by the 202nd Engineering Combat Battalion for US soldiers of the 82nd Airborne Division before D-Day.