Casey, O'Casey and MacCasey come from the Irish cathasach meaning vigilant in war, a personal name which is quite common in early Ireland. This no doubt accounts for the fact that O'Cathasaigh arose as a separate surname in at least five distinct areas in countries Cork, Dublin, Fermanagh, Limerick and Mayo with Mac Cathasaigh confined to the Louth/Monaghen area. In Medieval times the Dublin and Fermanagh Caseys were the greatest and most prominent though their power like that of other families had been broken by the seventeenth century. The name is still common in north Connacht generally. However, most present day bearers of the surname are to be found in Munster and not only Cork and Limerick but also in Kerry and Tipperary.