The Sweeney Name
Peter Cuffley has researched the ancient origins of our Sweeney branch.
By
way of summary, The Mac Suibhnes- anglicised as MacSween(e)y,
McSween(e)y, Swee(e)ny, MacSween, with up to 30 earlier variants, such
as McSwiny, M'Sweeny, etc - were fierce, heavily armed fighting men,
emanating from the Scottish Islands and Highlands.
The
MacSweeneys, probably crossing over from Argyle, arrived in County
Donegal in the 14th century, according to most authorities, where they
were hired by chiefs such as the O Donnell. Doe Castle was the
strongold of the MacSweeneys in County Donegal. Read more at http://tomgallen.com/2013/12/doe-castle-stronghold-of-the-macsweeneys/.
This
picture, from a 1924 publication called 'Hutchinson's Britain
Beautiful' (Volume 2), shows McSwyne's tombstone in Doe Castle.

From Donegal, they travelled to and settled in Cork, Sligo and Tipperary.
Our
Sweeney family springs from the group who travelled to Tipperary to
fight for the Butlers. There were already Sweeney families in the
Barony of Ikerrin in the 16th century. Cullahill was in that area.
You can read much more about the MacSweeneys at https://www.ireland101.com/tribe/MCSWEENEY.