Castlereagh (from Irish An Caisleán Riabhach, meaning 'the grey castle')[1] is the name of a former barony in County Down, present-day Northern Ireland.

[2] It spanned the north-eastern area of the county bordering the baronies of: Ards to the east; Belfast to the north; Iveagh to the west south; and Dufferin, Kinelarty, and Lecale to the south.

By 1841 the barony was divided into Castlereagh Lower and Castlereagh Upper.

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