I AM INDEBTED TO MAURA O'HANLON OF MOURNEABBEY, CO. CORK, A GREAT-GREAT-GRANDDAUGHTER OF 'UNKNOWN' SULLIVAN, FOR HER GENEROSITY AND HELP WITH MY O'SULLIVAN RESEARCH.
It is believed that 'unknown' was born in the townland of Caheraveelane in the civil parish of Kilshannig and in the Catholic parish of Glantane (near the village of Nad or Nadd) in Co. Cork. The Sullivans from Caheraveelane were known as the 'Caher' or 'Cahera' (pronounced 'kawra') Sullivans , presumably to distinguish them from the other Sullivan families in the area. They used the surname 'Sullivan' prior to the early 20th century when they started using the name 'O'Sullivan which would have been their ancestors' original name. Note that Caheraveelane seems to be spelled nowadays as Caheravellane.
It is possible that 'unknown's name was Thomas as the Tithe Applotment records of 1823 tell us that a Thomas Sullivan, the only Sullivan listed for Caheraveelane, occupied 16 acres of land in that townland.
I have been told that Sonia O'Sullivan, the well-known Irish athlete, is a descendant of the Caher Sullivans (although her O'Sullivan grandfather was apparently a native of Dublin) but I have been unable to verify this. I suspect there is no connection but would love to come across information that would indicate that there is in fact a link. Interestingly, several other descendants of the Caher Sullivans have been talented sportspeople, e.g., Jerry O'Sullivan who enjoyed success in boxing in Britain prior to emigrating to Australia; his brother Eugene (1927-1967), a distance runner who was selected to represent Australia at the 1956 Olympics; and Arthur Sullivan (born in 1884 in Caheraveelane), another fine distance runner.
The O'Sullivan surname is the third most common surname in Ireland, with four out of five O'Sullivan families in the country living in counties Cork and Kerry where O'Sullivan is the most common surname. The original Irish surname was 'O Suileabháin', deriving possibly from 'suil dubh án' meaning 'dark-eyed'. It is possible that the O Suileabháin families originated in South Tipperary from where they were driven out in the 13th century by the O'Briens and the Normans. Most settled in west Cork and Kerry. By the end of the 14th century, the O Suileabháins had split into at least seven different groupings, the most impportant of which were (i) Clann Gilla Mochuda of south Kerry, who in the 16th century, changed their name completely to McGillycuddy, (ii) the O'Sullivan Mór, based on the shores of Kenmare Bay, and (iii) the O'Sullivan Beare, rulers of the area around Bantry and of the Beara peninsula of Co. Cork. I am presuming therefore that my O'Sullivan family is descended from the O'Sullivan Beare.