O Longain, also anglicised as "Langan", arose initially in Co. Armagh, but quickly spread throughout the northern counties, and is now most common in Ulster in Co. Donegal. It seems likely that it shares its probable linguistic origin with O Longaigh, deriving from long, "ship", and therefore meaning "seafarer".
O Longaigh arose in the south of the country, in Co. Cork. The earliest records of the family, dating from the fourteenth century, show them as prosperous hereditary occupiers of church lands in the parish of Cannovee, in the barony of Muskerry in mid-Cork. This, together with the neighbouring parish of Moviddy, is the area with which the family remain most strongly associated down to the present. They took part on the losing side in the wars of the seventeenth century and, like virtually all of the native aristocracy, lost their possessions. Unlike most of the others, however, the descent from the last duly inaugurated Chief of the Name, Dermod O'Longy, remains unbroken. The official title is "O'Long of Garrenelongy", referring to a townland in the parish of Moviddy, and the current holder, officially recognised by the Chief Herald of Ireland is Denis Long.