Ó Cathail is an Irish surname that translates to "descendant of Cathal," with "Cathal" meaning "battle ruler" in Old Irish. This surname is associated with several notable families in Ireland, particularly in the province of Connacht. The Ó Cathail clan historically held power in regions such as County Galway and County Roscommon. Over time, the name has evolved, and variations can be found, including O'Cathail and O'Cathal. Today, individuals with this surname can be found both in Ireland and among the Irish diaspora worldwide.