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.
Danesfort 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). The name reflects a common belief up to the 19th century that many prehistoric monuments were the work of Vikings.