Castlereagh Lower (named after the former barony of Castlereagh) is a historic barony in County Down, Northern Ireland.

[3] The barony roughly matches the former Gaelic territory of Uí Blathmaic, anglicized Blathewic.

[4] 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.

[2] Castlereagh Lower is also bounded by Belfast Lough to the north and Strangford Lough to the south-east.

Below is a list of settlements in Castlereagh Lower:[5] Below is a list of civil parishes in Castlereagh Lower:[5][6][7]