In Irish Súileabháin means 'one' or maybe, 'hawk-eyed'. Their ancestry is from Olioll Olum, third-century King of Munster who was progenitor of the great Eoganacht clan. Tipperary was the first O'Sullivan territory, and as they multiplied they spread to Cork and Kerry. The senior chieftain, O'Sullivan Mór, had his stronghold at Kenmare Bay in Kerry. O'Sullivan Beare occupied Dunboy on Bantry Bay.

The O Sullivans fought at Kinsale and defended Dunboy to the last stone. Then began O'Sullivan Beare's epic march. In January 1602, with 400 soldiers and 600 civilians, they walked 200 miles north in two weeks of bitter cold and savage attacks to reach Brian O'Rourke's Leitrim Castle. Only 35 had survived, including one woman. Although Elizabeth was dead, James I was no friend of the Irish. O'Sullivan Beare and his family left for the hospitality of Philip III of Spain, where many of his family gave distinguished service in the army and navy.

In France, during the Revolution, an O'Sullivan was one of the chief tyrants, sinking barges full of priests and aristocrats to bypass the slowness of the guillotine!

There was a remarkable flowering of literary talent in the eighteenth and nineteenth centuries. Owen Roe O'Sullivan, teacher, sailor and womanizer, is now acknowledged to be a great Iyric poet, while Sir Arthur O'Sullivan was the musical half of the Gilbert and Sullivan operetta partnership.