This Irish surname was first anglicized O'Hagherin, which is a fair phonetic approximation of the original Gaelic O'hEachtighearna, derived from the Irish words each a horse and tighearna a lord. Later this was corrupted to O'Ahearne and finally the prefix O was dropped.
The O'Ahernes were originally a Dalcassian sept and up to the middle of the fourteenth century they were dynasts of Uí Cearnaigh, their territory being in the neighbourhood of Sixmilebridge, Co., Clare, not far from the city of Limerick. The name of Uí Cearnaigh is still locally preserved in that of the river Ogarney, which passes through Sixmilebridge, and falls into the Shannon near Bunratty. The proprietors of this territory (Ui Cearnaigh), previously to the yeare 1318, were the O’Eichtigherns, now Anglicised Ahern. In the course of time they migrated southwards and in 1659, when Petty's census was taken, the name was numerous throughout Counties Cork and Waterford. At the present time it is almost confined to Counties Cork and Limerick; but in the form Hearn and Hearne it is also well established in Co. Waterford. In this connexion it should be observed that Hearn is a fairly common indigenous name in England, so that persons so called in Ireland may be immigrants from that country. The Bishop of Kerry from 1336 to 1347 is described in the records as Alan O'Hathern alias O Hachierane.
It appears from MacNamara’s Rental that it comprised the parish of Kilfinaghta and part of the district lying between that parish and the city of Limerick. The Ogarney flows through the midst of the district, from the castle of Enaghoflinn to that of Rossmanagher. After that the river forms the boundary between Uí Ainmire and Tradraighe. It was an ancestor of O’Ahern, an ancient chief of this territory, who granted to St. Munchin the Island of Inis Sibtond, now the King’s Island, near Limerick. It does not appear that all the country reaching from Limerick to Kilmurry-na-gaull was included in the lands of the Uí Cearnaigh, since we have the authority of O’Heerin for placing two other chieftains in this district namely, the Uí Ainmire and Uí Sedna. The probability is that the territory of Uí Cearnaigh extended from the mountain of Sliabh Oighidh-an-Righ or Glennagross mountain to the parish of Kilmurry, and that the country stretching thence to the Shannon was divided between the Uí Ainmire and the Uí Sedna.
The Gaelic name O hUidhrin, that of an Offaly sept, has also been anglicized Hearne, Heron etc., though Heffron and Haveran are more usual forms in English. Giollananaomh OhUidhrin (d. 1420), who completed O'Dugan's celebrated "Topographical Poem", is usually O'Heerin in English.
Derry-Agherton, Portstewart, O' Hatheran
Hegarty, sometimes O'Hegarty but seldom Haggerty in Ireland (a form of the name found among Irish-Americans), is in Irish O hEigceartaigh (eigceartach means unjust). Though now associated principally with Co. Cork, the Hegartys of Munster are in fact a branch of the main O'Hegarty or Cenél Tighearnaigh sept of the Cenél Eoghan which was located on the borders of the present counties of Donegal and Derry.
In the fourteenth century the barony of Loughinsholin (Co. Derry) was their principal habitat; in the seventeenth they were more numerous in Tirkeeran (Co. Derry) and Inishowen (Co. Donegal) in the north, and in the baronies of Barrymore and Carbery West in Co. Cork. As is usually the case the present representatives of the sept are to be found in their traditional homeland and are to-day most numerous in Counties Cork, Donegal and Derry. The Ulster sept were subfeudatory to Ó Neill's army. The 1691 attainders of O'Hegartys relate to those of Ulster. The records of persecuted priests in the seventeenth century also indicate the Ulster character of the sept up till modern times.