There is little literature on the origin of this surname.

Leading authorities suggest that it is derived from the Gaelic Ó h-Íarnáin, a branch of Uí Fiachrach sept of the Aran Islands in County Galway.

It is generally found in Galway and, until recently, the Inishowen peninsula in Donegal. There is a strong oral tradition in Inishowen that the name is an anglicisation of the Spanish surname Hernandez, introduced by shipwrecked soldiers and sailors of the ill-fated Spanish Armada of 1588. However, this will remain a tradition, as there are no written records to provide evidence that this is the source of the name.

Just by co-incidence, though, most of the Armada ships that sank in Irish waters were wrecked off the coasts of ....... Galway and Donegal!