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.[3][4] The castle was ruined by the 1750s.[3]
Castlereagh Presbyterian Church was founded in 1650, with a building on Church Road from 1720, and the present one built in 1835.[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.[4]
Population of townlands was published at censuses from 1841 to 1926.