From The Irish Fireside, Volume 2, Number 29, January 14, 1884
This family, whose descent is the same as that of the great M`Carthy sept, which we gave last week, derives its name from Suileabhan, a celebrated chieftain of the tenth century.
It is divided into three branches by Irish genealogists: O'Sullivan of Beara, near Bantry, in the county of Cork; O'Sullivan Mor, Lords of Dunkerron, in Kerry, and the O'Sullivans of Knockgraffan, in Tipperary.
In the reign of James I. their estates were confiscated in revenge for their adherence to the Earls of Desmond and Tyrone in the Elizabethan wars; and the leading members of the family retired to Spain, where they contributed many distinguished officers to the Spanish service.
The illustrious Marshal M`Mahon is descended from the O'Sullivan of Beara on the maternal side. His grandfather, John M`Mahon of Autun in France, applied to the Ulster King-at-Arms to have his genealogy recorded in 1749, so as to leave with his posterity sufficient proof of the proud fact that they were of Irish descent.
ARMS OF O'SULLIVAN
Per pale vert; a stag passant;
boar passant, per pale, sable and argent in chief;
or, two lions combatant, supporting sword wreathed with serpent.
Crest—a robin on coronet.