"O'SULLIVAN, (p. 270) - In Irish O'Sullivan is O'Suileabhain. The derivation of the name is in dispute among scholars. There is no doubt that the root word is suil (eye), but whether it is to be taken as one-eyed or hawk-eyed must be left an open question. While not quite as numerous as Murphy and Kelly, Sullivan, which is by far the commonest surname in Munster, comes third in the list for all Ireland. Almost eighty per cent of the Sullivans (or O'Sullivans) in Ireland to-day belong to the counties of Cork and Kerry, the remainder being mostly of Co. Limerick, or of the city of Dublin, in which, of course, families from all the four provinces are found. Thus the O'Sullivans, as is almost always the case with the great Gaelic septs, are still concentrated in or near their ancient homeland. It was not until after the Anglo-Norman invasion that the O'Sullivans came to the fore. Their origin, however, is illustrious: descended from Eoghan (Owen) Mor, the father of the famous Oilioll Olum, they were, with the O'Callaghans, the MacCarthys and the O'Keeffe, one of the leading families of the Munster Eoghanacht. Some at least of them were lords of a territory near Cahir prior to the invasion: from 1200 onwards, however, they are to be found in the extreme south-west of Munster. There they became very numerous and powerful, dividing into a number of branch septs of which O'Sullivan Mor and O'Sullivan Beare were the most important. The former had his principal castle at Dunkerron on the shore of Kenmare Bay, the latter was lord of the modern baronies of Beare and Bantry. Though seldom appearing in any of the Annals before the year 1400, they were prominent in the sixteenth century."
The standard naming practice for sons among families in the area was for the first son to be called after the paternal grandfather, the second son after the oldest uncle (often the paternal great grandfather) and the third son after the father. In this way you get names repeating in each family over and over and indeed multiplying every generation. This, allied to the fact that there were just so many O'Sullivans in general, also gave rise to family nicknames.
About 1600 BC, a Celtic tribe led by a man named Owen landed on the Beara Peninsula in the southwestern part of Ireland. They call themselves the "Children of Owen" or the Eoganacht. As the group becomes larger, they are led by the McCarthy sept and form what is known as Munster (as in the four nations of Ireland: Ulster, Leinster, Connaught and Munster).
By 700-900 AD, the leading sept, the McCarthy-Mor split and a group adopted the name O'Suileabhain which is Irish Gaelic for the descendants of "One-Eye" (why is not known, possibly to scare away rivals) or "Eagle-Eye". The original land split gave them land along the River Suir in Co Tipperary near Knockgraffon. During this period the O'Sullivans of Knockgraffon lived quietly along the River Suir "peacefully stealing their neighbors' cattle and gold smithing".
In 1192, this group of O'Sullivans were driven from Tipperary by the Normans and appeared to undergo a major attitude change as they were driven into the O'Driscoll's territory. The O'Sullivans drove the O'Driscolls into the sea around the Beara Peninsula and the sept split into two groups: the O'Sullivan Mor and the O'Sullivan Bear. "Mor" means main, large or big and appears here to mean something like "main" or "main-line" Sullivans and "bear" or "beara" refers to the Beara Peninsula. The O'Sullivan-Mor group moved northwest to the Iveagh or Kenmare Peninsula behind the "Gap of Dunloe" in a stronghold from which they controlled the peninsula for almost 500 years of warfare before the English could penetrate the Gap. In this area, which is today called the Ring of Kerry, the O'Sullivan-Mor regarded themselves almost as a separate country. They had a castle at Kenmare called Dunkerron. The O'Sullivan Bear moved into the Bantry Bay area and had a castle at Ardloe.
Between 900 and 1609, an O'Sullivan-Mor led the Eagonnacht nine times and was its last leader in 1609. While the leader of the Eagonnacht, the Rock of Cashel was their castle.
In the 1300s there was a convention to determine which families in Europe were Royal. Families from all over Europe sent representatives. Most were turned down. How the O'Sullivan Mor was able to convince the committee that the O'Sullivan-Mor were a Royal Family is unknown, but he did it and as a result the O'Sullivan Mor coat of arms has the crown of a king on it.
In the nine years war culminating in the Seige of Kinsale on Christmas Eve of 1601, O'Sullivan Beare's forces were defeated and he returned to his castle at Dunboy. In June of 1602, Dunboy Castle was beseiged by Carew of England, and all of its inhabitants slaughtered; however, Donal Cam O'Sullivan Beare at the time was at another castle in Ardea, awaiting reinforcements from Spain. In January of 1603 he led 1,000 followers across Ireland in what is known asO'Sullivan's March. Thirty-five (35) people finished the journey, and Donal eventually escaped to Spain. Other Irish Chieftans escaped to Spain in what is known as the "Flight of the Earls", except one: the O'Sullivan Mor was driven back to Dunloe. When the O'Sullivan Mor fell back into the Ring of Kerry, he disappeared.
The O'Sullivan Bear was killed while attempting to save a friend during a duel in Spain in 1618. Some of his descendants, however, were given land grants in the land of Beara and Bantry. The present day O'Sullivan Bear clan claims leadership of the O'Sullivan clan world-wide. No claim for the title of O'Sullivan Mor has been honored since 1609.
By 1601 and the Seige of Kinsale, there were seven O'Sullivan septs (per Irish surnames by Wolfe) including O'Sullivan-Mor (representing 40% of all Sullivans) of Co Kerry and areas north of Kerry, and Co Tipperary and O'Sullivan-Bear of Co Cork (30% of all Sullivans). About thirty years after this, Cromwell came to Ireland and the Sullivans were one of his favorite targets in his campaigns. Migration out of the British Isles accelerated as people tried to get out of his way.
Today the name of Sullivan is so common in some areas of Ireland that nicknames designate the families. The family nicknames don't have anything to do with -Mor or -Bear, but are based on location. In theory, all Sullivans are descended from one man, making the name O'Sullivan one of the oldest, most numerous European family surnames (differing from occupation surnames like Smith).