In Ireland the system of hereditary surnames began earlier than in most other European countries. Irish surnames evolved over many hundreds of years, and developed out of a more ancient system of clann name.

The Irish septs which grew out of these clanns came to be identified by their surname. The bearers of a particular surname occupied a particular territorial unit. Notwithstanding the many confiscations of land in Ireland·s history, the bearers of any particular Irish surname in most case still inhabit those areas their ancestors occupied prior to such confiscations.

The clann names were formed from the name of distinguished ancestors, and descendants occupied particular areas. It is important to remember that the history of these clanns evolved over at least five centuries, from the fifth or sixth century to the eleventh century.

Ui Niall, descendants of Niall of the Nine Hostages in Meath and west Ulster; these divided into the Northern Ui Niall and the Southern Ui Niall. The Ui Neill by the seventh and eight centuries dominated the northern half of Ireland, which was known as Leth Cuinn (Conn's Half).

The Northern Ui Neill claimed descent from Conall Gulban, Enda, and Eoghan, and ruled Aileach. The Southern Ui Neill claimed descent from Coirpre, Loegaire, Fiachu, Maine, and Conall Cremthainne, and ruled Tethbae, Mide and Brega.

Ui Briuin, descendants of Niall's brother Brian. They ruled in Connacht and subsequently a sub-branch the Ui Briuin Breifne expanded into what is now Leitrim and Cavan (Breffny).

Ui Fiachrach, descendants of Fiachrach, also a brother of Niall; these divided into the Ui Fiachrach Muaidhe in north Mayo and Sligo and the Ui Fiachrach Aidhne in south Galway.

Ui Maine, descendants of Maine Mor in Connacht.

Dal Cais, descendants of Cormac Cas in Munster.

Eoghanacht, descendants of Eoghan Mor in Munster.

As time passed perhaps during the seventh or eight centuries, the clanns split into distinctive smaller groups, who inhabited their own particular kingdoms. These small kingdoms were known as Tuath and there were probably 150 of them in existence at any given time.

Each Tuath had its own Ri (king). Any increase in the number of Tuath brought about an increase in the number of dynastic families. Every Irish Ri was elected from within the royal fine of his own Tuath, usually from the derbfhine. These were the descendants in the male line from a common great-grandfather. Practically all such Rô were subject to the overlordship of another Ri, at the top of the pyramid was the Ard-Ri.

However too much must not be read into the possible authority of an Irish medieval High Kingship.

Gaelic society further evolved between the eleventh and twelfth centuries. Those who were descendants of a common ancestor, and inhabited the same locality came to be known as septs. The area of land controlled by a sept was know as a ballybetagh (and may have been co-extensive with the later parish), which was composed of sixteen ballyboes. A ballyboe was an area of land which could support a number of families, the modern equivalent is a townland.

The overlord (himself being the chief of a sept) of a number of septs perhaps would have had the lordship of an area the size of a barony. Above such an overlord would have been another overlord the status of O’Neill or O’Domhnaill. The sept system was adopted by those who arrived during the Cambro-Norman invasion. The members of these septs were designated by a common surname.

What is not readily understood, when we look at how numerous many Irish surnames are, is that the bearers of a particular surname may be descendant of distinctly different septs. Also it often happened that sub-septs were formed, and quite different surnames were adopted by the descendants of a common ancestor.

Commenting upon how numerous were the members of some septs, Dubhaltach MacFirbhisigh wrote in 1650 ´For it is a usual thing in the case of great princes, when their children and their families multiply, that their clients and followers are squeezed out, wither away and are wasted.

Eoin MacNeill suggested that a reason for this was the Law of Debad which had the effect of passing an increased amount of land to an overlord in the absence of direct heirs. The septs were an important part of the organisation of Gaelic society.

In Gaelic Ireland the bonds which cemented society were the duties and rights attached to blood relationship. The sept system did not survive the colonisation of Ireland during the seventeenth century. It could not do so in the absence of the Brehon Law or the Gaelic leadership.

Before surnames becoming fixed and hereditary, individuals were designated other than by their own personal name. From an early period a personal name derived from a father (patronymic) or male ancestor came to be used to particularize the individual. Irish patronymics surnames were formed by prefixing Mac· (son of) to the genitive case of the fathers name, or Ua· or O· (grandson of) to that of the grandfather. The Annals are full of such designations. The practise of forming surnames with Ua· or þ· had ceased before the Cambro-Norman invasion. Mac· surnames are generally of a later date than þ· surnames. In time the patronymic, which before was purely personal and changed with each generation gradually became fixed like the clann names centuries before, and began to assume the permanent and hereditary character of a family surname. The period at which this change began can only be determined approximately. The eleventh and twelfth centuries must be assigned as the period within which Irish patronymic surnames became fixed and hereditary.