This cadet branch was founded by Dermot MacCarthy, 1st Lord of Muskerry, second son of Cormac MacCarthy Mor, King of Desmond,[10] who was in 1353 created Lord of Muskerry by the English.

The family's ancestral lands of were situated along the River Lee in the baronies of Muskerry West and Muskerry East, in central County Cork west of the City of Cork.

Following the forfeiture by Donogh McCarthy, 4th Earl of Clancarthy, the castle property passed to the Hollow Sword Blade Company who subsequently sold it to Sir James St. John Jefferyes, Governor of Cork in 1688.

[13] The titles of Mountcashel and Baron Castleinch, of the (1689 creation, went extinct with the death of Justin MacCarthy in 1694.

At that date the MacCarthys of Muskerry had therefore lost all their noble titles in the peerage of Ireland.