The family name Regan, along with its cognates O'Regan, O Regan, Reagan, and O'Reagan, is an Anglicized form of the Irish surname Ó Riagáin or Ó Ríogáin, from Ua Riagáin. The meaning is likely to have originated in ancient Gaelic ri "sovereign, king" and the diminutive suffix -in; thus "the king's child" or "big king". The name was borne by two distinct families: one seated in Meath, the other in Thomond.
