The Sweeneys trace their ancestry back to Niall of the Nine Hostages, a 5th-century High King of Ireland. The sept was very much of Donegal origin, but it wasn't until the 14th century that three great septs of the Tirconnell MacSweeneys were established. More than a century later, a branch of the clan went to Munster, and today the name is quite widespread in counties Cork and Kerry.

The principal seat of the MacSweeneys in Donegal is Doe Castle, near Creeslough in north Donegal, dating from the early 15th century. Rahan Castle, near Killybegs in Co. Donegal, was another MacSweeney clan stronghold.

The present head of the Fanad, Co. Donegal, Sweeneys is Chevalier Loughlin Sweeney, a Dublin bank manager. The head of the family in Spain is Dona Mary de Navascues, the Marquisa de Casacagal. Her Sweeney forebears were Spanish governors of Cuba. In Canada, the Sweeney clan is represented by Captain Richard Mingo Sweeney, whose family emigrated from Co. Tipperary in 1830.