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.,_Duke_of_Magenta