Clanwilliam House was built for Samuel Barbour, a linen thread manufacturer in 1864 by William Joseph Barre. Clanwilliam is a barony in Co. Tipperary. The name was changed in the 1870s to Danesfort House. The building now houses the US Consulate-General in Northern Ireland.
The house was built on what had previously been known as Pleasure-House Hill - apparently on the site of an old rath or fort. During the progress of excavating the ground for the building, several ancient urns and celts or hatchets were found. They were subsequently mounted and exhibited in cabinets in the library by the first owner of the house (Belfast Telegraph, 04/03/2004). These items, known as the Malone Hoard, included a number of very large polished stone axe heads, which are now displayed in the Ulster Museum. They are made of porcellanite, a rare mineral only found in Ireland at two locations, both in north Co. Antrim. They are believed to be ritual items since they are too heavy to have been of practical use.
The name Danesfort reflects a commonly held belief up to the 19th century that many prehistoric monuments were the work of Vikings.