[2] About 1350, at the site of a ráth in the Castlereagh Hills, Aodh Flann O’Neill is said to have built the castle from which the townland was named.
[3] Aodh was of the Clandeboye, a branch of the O'Neill dynasty who colonised the area from the west.
[3][4] Con MacShane O'Neill raided Belfast from the castle after Christmas 1602, leading to retributions from the Elizabethan settlers there.
[3][5] In 1615, he was reduced to selling the manor comprising the castle and grounds to Moyses Hill, ancestor of the Marquesses of Downshire, who still exercised jurisdiction there in the 1840s.
[5][6][3] The title of Viscount Castlereagh was created in 1795 for Robert Stewart, Baron Londonderry; when he was promoted to Earl of Londonderry in 1796, "Viscount Castlereagh" was the courtesy title of his son, infamous for suppressing the Irish Rebellion of 1798.