The earliest reference to the clan Ó Haichir (now O'Hare) is to be found in the Annals of the Four Masters.
It is the name of an Oriel family who were chiefs of Oirtheara, now the baronies of Orier, in the east of County Armagh.
According to the traditional Irish pedigree, the family is descended from Slioch Ir, who was the son of Ior and the grandson of Cathal Ruadh, who was killed in 1401.
[citation needed] O'Hare is one of the few surnames which resisted the general tendency in the 18th century to discard the "Ó" in their name.
[citation needed] (The creation of societies such as the Gaelic League in the late 19th century resulted in the widespread resumption of the "Mac" and "Ó" prefixes to many Irish surnames.)