Castlereagh Lower (named after the former barony of Castlereagh) is a historic barony in County Down, Northern Ireland. It was created by 1841 with the division of Castlereagh into two. It is bordered by three other baronies: Ards Lower to the east; Dufferin to the south; and Castlereagh Upper to the west and south-west. Castlereagh Lower is also bounded by Belfast Lough to the north and Strangford Lough to the south-east.