Viscount Dillon, of Costello-Gallen in the County of Mayo, is a title in the Peerage of Ireland. It was created in 1622 for Theobald Dillon, Lord President of Connaught. The Dillons were a Hiberno-Norman landlord family from the 13th century in a part of County Westmeath called 'Dillon's Country'. His great-grandson, the seventh Viscount, was a supporter of the Catholic King James II of England and was outlawed after the Glorious Revolution. He founded 'Dillon's Regiment' of the Irish Brigade in the French Army, which was supported by the Wild Geese and achieved success at Fontenoy in 1745.

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.