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The Gaelic root of O'Sullivan is Suileabhan - with suil meaning "eye," dubh possibly "black" or "dark," and the diminutive an acting as a suffix.  Some think Suileabhan means one-eyed, others hawk-eyed.  Suileabhan was a person - born in 862 and a descendant of Finghin the King of Munster in the 7th century - who is seen as the first of the O'Sullivan line.  Many consider that the O'Sullivan clan represents the most senior bloodline of the Gaelic families.

Select O'Sullivan Resources on The Internet

• The International O'Sullivan Clan.  Clan association.
• The Book of Sullivan.  Sullivan history and timeline.
• The O'Sullivan Network.  O'Sullivan information.
• O'Sullivan History.  Clan history and miscellania.
• Branches of the Sullivan Family.  Early Sullivans from Ireland to Virginia.
• The Sullivan Family.  Sullivans from Ireland to Australia and San Francisco.

Ireland.  The original home of the O'Sullivans was Tipperary.  They were driven westward from these lands by the Anglo-Normans in the 1190's and divided into two septs:

• the O'Sullivan Mor (the larger or the greater) in south Kerry
  
• and the O'Sullivan Beare (from the Beara peninsula) in west Cork.  
  

These septs prospered until the 1590's when they came up against the English.  In a final confrontation in 1601 Donal O'Sullivan Beare had taken command of the Irish Munster forces and, with their Spanish allies, they faced the English at Kinsale.  The result was defeat.  The O'Sullivan Beare fortress of Dunboy castle was then lost.  Gary Sullivan's 2007 book History of the O'Sullivan Clan recounts these events and the overall clan story.

Today Kerry and Cork account for about 80 percent of the O'Sullivans in Ireland, with numbers also in Limerick and Dublin.

Elsewhere.  After the Kinsale debacle and the travails of Irish families in the following centuries, many O'Sullivans left.  John O'Sullivan of the O'Sullivan Beares was an early arrival in America, coming to Virginia in 1655 as a planter.  There are now more than three times as many Sullivans and O'Sullivans outside Ireland than within.  The "O" in O'Sullivan has been retained or restored in Ireland generally, but has dropped off elsewhere.

Numbers (000's)	O'Sullivan	Sullivan	Total
Ireland	37	9	46
UK	2	33	35
America	3	57	60
Elsewhere (1)	-	18	18
Total	42	117	159

(1) Canada, Australia, and New Zealand.

Donal O'Sullivan was the last independent chief of the O'Sullivan Beare sept (before defeat by the English).
Owen Roe O'Sullivan, who wrote in the 18th century, is considered the last of the great Gaelic poets.

Alexander Sullivan was the editor and proprietor of The Nation, a proponent of Irish nationalism in the 19th century.
Louis Henri Sullivan, an architect working in Chicago in the 1880's, is considered the father of the modern skyscraper.

John L. Sullivan was the first recognized heavyweight boxing champion.
Arthur Sullivan was the composer of the Gilbert and Sullivan operettas. 
Ed Sullivan was the American host of the TV Ed Sullivan Show.
Ronnie O'Sullivan, the "Rocket," is considered the best snooker player in England today.