Ireland was one first countries in which a system of hereditary surnames arose: they were known in Ireland long before the Norman invasions brought English-style surnames to the country. The Irish prefixes Mac (son of ) and O (grandson or descendant of ) gave rise at an early date to a set of fixed hereditary names in which the literal patronymic meaning was lost or obscured . O'Neill, for example, is known as a hereditary byname from the 4th century onwards These surnames originally signified membership of a clan, but with the passage of time, the clan system became less distinct, and surnames came to identity membership of what is called a 'sept' a group of people all living in the same locality, all bearing the same surname, but not neccessarily descended from a common ancestor. Adoption of the name by people who did not otherwise have a surname and by dependents was not uncommon. Later, nicknames were in some cases to supersede the original clan names, giving rise to surnames like Duff 'Black'
Considerable caution is called for in interpreting names in early Irish records. MacLysaht points out that, among the successors of St Patrick as bishops of Armagh, Torbac MacGormain (d .812)and Diarmuid O Tighearnaigh were not members of the Gorman and O'Tierney families, but respectively son of a man whose baptismal name was Gorman and grandson of one called Tierney.
Just over one hundred years after the Norman Conquest of England, the first Normans arrived in Ireland. Richard de Clare, Second Earl of Pembroke (d 1176) known as 'Strongbow', was invited to Ireland Dermot Mac Murrough, King of Leinster, whose daughter he married , to help him in his wars with his neighbours. He was accompanied by several retainers whose names, like his own, have become well estadlished as surnames in ireland, among them Maurice Fitzgerald, son of Gerald, steward of Pembroke Castle. The Normans established themselves in Leinster and paid homage to Henry II of England. Some of the Normans settlers in Leinster acquired surname derived from Irish. Thus, de Bermingham became Mac Corish and Nangle became Costello. The process in these two cases was as follows. The Norman first name of Piers de Bermingham was Gaelicized as Feoras: his son was known as Mac Fheorais (son of Piers) which was later Anglicized phonetically as Mac Corish (fh being silent in Irish). Livewise, the son of Oisdealb de Nangle was known as Mac Oisdealbhaigh, Anglicized as Costello.
In the latter part of the 16th century, another influx of settlers arrived under the patronage of Elizabeth I of England, and colonized the country beyond the 'Pale', the area around Dublin that was the only part firmly under English control. Typical of these was Richard Boyle (1566-1643) who gained control of munster and was created Earl of Cork. The surname Boyle existed in Ireland before his arrival, and his original family name appears to have been de Binville. It is not clear how and when he acquired the distinctively Irish surname Boyle. His seven sos ensured that this surname would be well settlers were encouraged to migrate from Scotland to Ulster, thus establishing the distinctively Scots surnames of Protestant Ulster.
During the long centuries of English domination, Irish surnames were crudely Anglicized. The Irish Gaelic language was proscribed, or by translation. At its mildest, the prefixes Mac and O were abandoned, so that O Manachain became Monaghan and Mac an Fhailghigh became McNally and then Nally. At worst, Irish surname were distorted beyond all recognition. Thus, Mac Giolla Eoin son of the servant of Eoin was transmuted into Munday by confusion of the last past of the name with Irish Luain, genitive of Luan Monday. O Glasain became Gleeson, while O Beaglaoich became Begley. Only in recent years, since Irish independence in 1921, has a reversal set in, so that Irish people are now adopting Gaelized forms of their names, even though they may not be able to speak Irish. Gaelicization of a surname has become a statement of national and political identity, so that some whose names are actually of Norman French or English origin nevertheless crate Gaelic versions of them.
Irish surname proper are never derived from placenames. The reverse is in fact the case. Ballymahon is a typical example of one of the many placenames derived from a surname its is named as the townland of Mahon'.
In the 19th century, political repression and famine combined to fore many Irish people to seek other countries in which to live. Large numbers emigrated to the United States, where strong emotional ties to Ireland are still preserved in many families, while others found themselves transported, willingly or otherwise, to Australia, often after having first tried to make a living in England. Irish surnames are now very widely dispersed, and are common in England as well as in Ireland, United States, and Australia.