In the Paddy O’Keeffe papers in the Cork Archives dealing with a query on the Symms family there is a letter from Edward Keane (National Library) of the 5th September 1961. He states that the Marshall and a few other famous McMahons are descended from Patrick McMahon and Margaret O’Sullivan who married in Bantry in 1707. He asks Paddy O’Keeffe for knowledge of this Margaret.
http://en.wikipedia.org/wiki/Patrice_de_Mac-Mahon,_Duke_of_Magenta
West Cork History 
Pingback: Spanish Knights of Irish Origin, The Count of Berehaven, Brigadier Daniel O’Sullivan, Governor of Coruna, born Bantry, West Cork, sponsor of Dionisio O Calaghan born Madrid 1718, Grandmother Margaret McCarthy, born Cork, Captain Daniel O Sulivan, Sp