Castlereagh, the chief residence of this latter branch of the [O'Neill] family - which probably stood on the site of the former Orange Hall (now Belmont Carpets) on the brow of Church Road - was perhaps first built in the late 14th century in what became the parish of Knockbreda, in Castlereagh in the hills overlooking Belfast. Its name is derived from the Irish an caisleán riabhach - the grey castle - and was, in 1552, described as standing on the plain in the midst of the woods of Dufferin". In 1744, it was located by Walter Harris [Go to further information for details about him] : 'Not much more than two miles east of Belvoir, and about as much south-east of [the Long] Bridge, are the ruins of an ancient castle, called Castlereagh, from whence the Barony of that name is denominated. It is situated on top of a hill... the fort has a fosse which encompasses three-quarters of it ... In the midst of the fort stood a castle, formerly the seat of Con O'Neill, proprietor of that large tract of country [224 townlands in all], which was afterwards divided among Lords Clandeboye, Ards and the ancestors of the present Lord Hillsborough". (Keith Haines, Martin Cooke, pp 3-4). This road takes its name from the Irish meaning “grey castle” in which Con O’Neill lived.
Note that the name Castlereagh Road was applied to a different road on James Williamson's map of 1791, later called Cregagh Road, and even later Woodstock Road. However, given that the road does not lead to Castlereagh townland and this name is not supported by other sources, it is possible that Williamson's information is unreliable in this regard.