Theobald Dillon, 1st Viscount Dillon, was an Irish military commander and adventurer. He held extensive lands in eastern Connacht and north-western Leinster, some acquired by sharp practices. He was a loyal supporter of Elizabeth I of England in her Irish wars.

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.