The Origins of Irish Surnames
Although up to the tenth century, surnames in Ireland were not hereditary, the influence of the church, dating from this period, can still be seen in many common modern Irish surnames, in particular those beginning with "Gil-" or "Kil-", an anglicised version of the Irish Giolla,
meaning follower or devotee. Thus Gilmartin, in Irish Mac Giolla Mhairtin, means "son of a follower of (St.) Martin". Similarly, the church is the origin of all of those names starting with "Mul-", a version of the Irish Maol,
meaning bald, and applied to the monks because of their distinctive tonsure. Thus Mulrennan (Ó Maoilbhreanainn) means "descendant of a follower of St. Brendan".
While many of the names appearing in accounts of this time appear similar in form to modern Irish names, incorporating in particular the prefix "Mac" (meaning "son of"), in fact they were not hereditary, lasting only one generation. Thus Turlough mac Airt, was Turlough, son of Art; his own son would be Conor mac Turlough, Conor son of Turlough.
Nonetheless, Ireland was one of the first European countries in which a system of fixed hereditary surnames developed. The earliest names appear to be those incorporating "Ó" or its earlier form Ua, meaning "grandson". According to Fr. Woulfe, an early authority on Irish surnames, the first recorded fixed surname is O'Clery (Ó Cleirigh), as noted by the Annals, which record the death of Tigherneach Ua Cleirigh, lord of Aidhne in Co. Galway in the year 916. It seems likely that this is the oldest surname recorded anywhere in Europe.
By the eleventh century many families had acquired true surnames as we would know them today. All of these surnames incorporate the same two basic elements, "O" or "Mac", together with the personal name of the ancestor from whom descent is indicated. In many cases this ancestor can be quite accurately identified, and the origin of the name dated precisely. Thus, at the start of the eleventh century, Brian Boru possessed no surname, being simply "Brian, High-King of the Irish", his grandson Teigue called himself Ua Briain in memory of his illustrious grandfather, and the name became hereditary thereafter. Similarly, the O'Neills derive their surname from Niall mac Aoiodh, who died in 917.
Due to linguistic changes, the origins of many of the personal names such as Niall or Brian which form the stem of the surname remain obscure, but two broad categories can be distinguished, descriptive and occupational. In the first category, we can guess that the progenitor of the Traceys (Ó Treasaigh) was a formidable character, treasach meaning "war-like", while the ancestor of the Duffs must have been dark-featured, since dubh, the root of the name, means black or dark. Among the occupations recorded in names are the churchmen dealt with above, clerks (Clery, Ó Cleirigh, from cleireach), bards (Ward, Mac an Bhaird, from bard), spokesmen (MacCloran, Mac Labhrain, from the Irish labhraidh), and smiths (McGowan, Mac Gabhainn, from gabhann). One category of name, common in English, which is extremely rare among Irish names is the toponymic, deriving from the name of a locality. It seems likely that this reflects the fact that, for the Gaels, who you were related to was much more important than where you came from.
Although the immediate reason for the early adoption of hereditary names in Ireland may have been a rapidly expanding population, it can also be seen as the logical outcome of a process at work from the times of the earliest tribal names. Originally, these indicated identification with a common god, often connected with an animal valued by the tribe, in the case of the Osraige, or "deer-people", for example. Next came identification with a divine ancestor, the Boandrige, for instance, claiming descent from the goddess Boand, the divinised river Boyne. Later the ancestor was merely legendary, as for the Eoghanachta, while later still the tribe claimed direct descent from a historical ancestor, as in the case of the Ui Neill. This slow emergence of kin-relationships out of religion and myth into the realm of history would seem to reach its logical conclusion with the adoption of hereditary surnames, permanent proof of verifiable ties of blood. On a more mundane level, of course, such proof was a valuable political asset, since it demonstrated membership of a powerful kin-group. Even today, the fact that all Gaelic names, without exception, begin with Ó or Mac is undeniable and continuing proof of the significance of family and kin for the Irish.
Although it began early, the process of the creation of surnames was slow, and continued for over six hundred years. As the population grew and new families were formed, they sought to consolidate their identity by adopting hereditary surnames of their own, usually by simply adding MAC to the first name of the founding ancestor. In the course of this process, then, many surnames were created which are in fact offshoots of more common names. Thus, for example, the MacMahons and the McConsidines are descended from the O'Brien family, the former from Mahon O'Brien, who died in 1129, the latter from Constantine O'Brien, who died in 1193. The continuing division and sub-division of the most powerful Gaelic families like this is almost certainly the reason for the great proliferation of Gaelic surnames.