Family History and Genealogy Resources by Surname
O'Callaghan Surname Origin
(Origin Celtic or Gaelic) The descendants or tribe of Callaghan, from Ciallach, prudent, judicious, discreet.
Source: An Etymological Dictionary of Family and Christian Names With an Essay on their Derivation and Import; Arthur, William, M.A.; New York, NY: Sheldon, Blake, Bleeker & CO., 1857.
O'Callaghan Surname Meaning and Family Facts
There is more to O'Callaghan family history than the origin of your surname:
A O'Callaghan Family History Thought:
'Every man is his own ancestor, and every man his own heir. He devises his own future, and he inherits his own past.' -- H. F. Hedge