Ó Cathasaigh is a Gaelic Irish surname, the original Gaelic form of the widely recognized surname Casey. It is a patronymic surname, meaning it denotes descent from a male ancestor. The name is derived from the given name Cathasach (also Cathassach), which in turn comes from Old Irish cathaisech, meaning ‘vigilant’ or ‘noisy’. The surname translates to “descendant of Cathasach.” Etymology and Linguistic Background In Irish patronymic surnames, the prefix Ó (sometimes anglicized as O’) means “grandson” or “descendant.” The root here is Cathasach, an Old Irish personal name formed from cath (battle) and possibly s-derivation meaning “watchful” or “alert.” The genitive form of Ó Cathasaigh is Uí Chathasaigh; the feminine equivalent is Ní Chathasaigh (daughter of an Ó Cathasaigh). Over time, the surname was Anglicized as Casey, which has become far more common in English-speaking contexts. History and Distribution The Ó Cathasaigh family was historically prominent in the Gaelic regions of Ireland, particularly in County Cork and County Kerry. According to Gaelic genealogies, the sept traces its descent from Cathasach, a son of Fingin, who was a descendant of Olioll Olum (a legendary 3rd-century king of Munster). They were part of the Dál gCais (Dalcassian) tribal grouping. Later, during the dissolution of the Gaelic order in the 16th and 17th centuries, many Ó Cathasaigh families were dispersed, and bearers began adopting the form Casey, especially after the requirement for English surnames during the Penal Laws. Today, both Ó Cathasaigh and its Anglicized form are most common in Ireland (especially Munster) and in areas of the United Kingdom, America, and Australia that experienced Irish diaspora migration in the 19th century. Contrast with the Given Name Casey It is important to distinguish the surname Ó Cathasaigh from the homographic given name Casey, which was retronymically created from the selfsame Irish surname. The given name has risen in use globally, with bearers such as American railroading hero Casey Jones (who himself derived the nickname from his apparent hometown of Cayce, Kentucky); however, that does not directly relate to the original Irish surname etymology. The spelling with ’ne’ is English, while Ó Cathasaigh is in agreement with official Irish surname maintenance efforts. Conclusion and Key Facts Meaning: descendant of the vigilant/noisy son (Cathasach) Origin: Irish Gaelic Diminutive/Anglicized form: Casey Patrilineal variant: O’Casey in English usage (standard writing: without apostrophe but the modern Ó is used more commonly among Irish and Bardic).