Costello is one of the ancient baronies of Ireland. Unusually for an Irish barony, it straddles two counties: County Mayo and County Roscommon. It comprises the modern day districts of Kilkelly, Kilmovee, Killeagh, Kilcolman, and Castlemore.

Ballaghaderreen is a town in County Roscommon, Ireland. It was part of County Mayo prior to 1898. It is in the north-west of the county, near the borders with counties Mayo and Sligo, just off the N5 road. The population was 1,808 in the 2016 census.