The surname is associated with the southwestern part of Ireland, and was originally found in County Tipperary and Kerry before the Anglo-Norman invasion.
The female form in Modern Irish is Ní Shúileabhán; "ní" is the shortened form of iníon uí, iníon "daughter", uí, the genitive of ó "grandchild, descendant".
In his book titled The Surnames of Ireland, genealogist Edward MacLysaght states that “while there is no doubt that the basic word is súil (eye) there is a disagreement as to the meaning of the last part of the name.” It is interpreted as súildubhán ⇄ “little dark-eyed one” by Woulfe in Sloinnte Gaedheal is Gall, from súil ⇄ "eye," dubh ⇄ "dark/black," and combined with the diminutive suffix -án.
Less common spelling variants of the name include: Sullavan, Sullivant, Sillivant, Silliphant, and Sillifant.
Some O'Sullivans in the midlands and south Ulster were originally (O) Sullahan (from Ó Súileacháin, probably from súileach, quick-eyed, according to MacLysaght).