Rathdown (Irish: Ráth an Dúin) is the south-easternmost barony in County Dublin, Ireland.

After the Poor Relief (Ireland) Act 1838, the poor law union of Rathdown covered a similar area to the Dublin and Wicklow baronies combined; after the Local Government (Ireland) Act 1898, the Dublin portion of the poor law union comprised the rural district of Rathdown No.

1 and the urban districts of Blackrock, Dalkey, Killiney and Ballybrack, and Kingstown (later renamed Dún Laoghaire).

[5][6] The barony of Rathdown also included part of the poor law union of Dublin South, comprising after 1898 parts of the rural district of Dublin South and the urban district of Rathmines and Rathgar.

Civil parishes wholly or partly within the barony are:[7] Booterstown, Dalkey (transferred from Uppercross in 1842[1]), Donnybrook, Kilgobbin, Kill, Killiney, Kilmacud, Kiltiernan, Monkstown, Oldconnaught, Rathfarnham (mostly transferred from Newcastle in 1842[1]), Rathmichael (two townlands transferred from Newcastle in 1842[1]), Stillorgan, Taney, Tully, Whitechurch.