There's an old saying that is supposed to answer quite neatly the question of which Irish surnames are "genuinely Irish"--that is, originally adopted by families of Irish Gaelic origin. It ends thus:

...and if he lack both O and Mac no Irishman is he.

There is also a widespread belief outside Ireland that the prefix Mc denotes an Irish Gaelic name, while Mac ultimately indicates Scots Gaelic descent.

In fact, these are both fallacies--or generalizations at best. The subject of Irish surnames is a complex one, largely because Irish history itself has been complex, marked by successive influxes of "foreigners"--from Vikings to Anglo-Normans, from "Old English" (Catholics) to Protestant English and Lowland Scots, with further leavening by gallowglasses (Scots mercenaries), Welsh, French Huguenots (Protestants), Palatine Germans and others.

Surnames that were adopted by the "native" Irish or brought from across the sea often did not remain unchanged for long. At various points in time, individuals or families who had come from England before Tudor times tended to adopt Gaelic culture, and changed their names accordingly. Later, as England's political dominion in Ireland expanded, many "genuine" Irish names were anglicized. They were either rendered into English by government clerks who usually had no feel for the Irish language, or changed by the holders themselves to gain economic advantage in an increasingly anglicized society.

The story of Irish surnames must thus be painted with broad brush-strokes. We shall here attempt to summarize some of the more common types of changes that occurred in names, and important events or forces that lay behind these changes.

Brian Boru (926-1014 A.D.), the first and only "high king" to unite the various petty kingdoms of Ireland into a single realm, is often credited with ordering the general adoption of hereditary surnames that seems to have occurred in the eleventh century. This claim has never been proven to be more than legend. In any case, about the time of Brian's reign, subjects who had began to adopt patronymic surnames. These were names based on the Christian name of their father, on the one hand, or the name of their grandfather or one of his paternal forebears, on the other.

A man, for example, whose father was named Niall could, if he wished, honor his father by adding the prefix Mac (or its shortened form, Mc) to his father's Christian name. Hence, he would be known as so-and-so Mac Niall--and his descendants might come to use a variant spelling such as MacNeill, McNeil, or McNeal, in some cases dropping the Mc or Mac prefix.

If this same man wished to name himself for his grandfather, Donnchadh, or perhaps Donnchadh's father, Conchur ( = Conchobair), he would prefix "Ua" to one of their names. (Note: The "Ua" soon evolved into "O," though at first without the apostrophe, which came about later under English influence.) Hence, Ua Donnchadha, or O Donnchadha, "grandson [or descendant] of Donnchadh," and Ua Conchur or O Conchur, which meant "grandson [or descendant] of Conchur." O Donnchadha would eventually be transformed into such anglicized versions as O'Donoghue, Donoghue, Donohoe, and Dunphy. From Ua Conchur, we would ultimately derive the Anglicized forms, O'Connor or simply Connor.

Although patronymics were most common, some of the Irish adopted surnames denoting their father's occupation. These, too, were passed down, generation to generation. A man whose father was a bard, or poet (Bhaird in Irish) might adopt the surname, Mac an Bhaird, son of the bard, which was later anglicized as Ward or MacWard. Similarly, another might adopt Mac an Ghabhann (son of the smith), which was later anglicized to MacGowan, and still later sometimes simply translated into its English equivalent, Smith.

At the time of the Battle of Clontarf in 1214, when King Brian had defeated the Danes (Vikings) and their Irish allies, there had already been colonies of Vikings in Ireland for some time. At least after their defeat, if not before, these new Irishmen of Norse roots also adopted patronymics. Hence, an Eric whose father was named Olaf might take the name, Mac Olaf, "Son of Olaf," which eventually came to be spelled McAuliffe.

The Anglo-Normans, who first arrived in 1169, quickly became quite Gaelicized, much to the chagrin of their overlords in England. As the phrase went, they often became "more Irish than the Irish themselves." Many, however, kept their own surnames, which might or might not contain the prefix, Fitz (from the French, fils, meaning son of). With the exception of the surname Fitzpatrick, any Irish or English surname that begins with Fitz today (e.g. Fitzwalter, Fitzgibbon, Fitzsimmons) is an almost sure sign of Norman descent in the male line.

Other Anglo-Normans brought names lacking the Fitz prefix that are still quite common in their evolved forms, such as Burke (originally de Burgo), Cusack and Roche. There are a few examples of Anglo-Norman families adopting Gaelicized surnames. In the wake of the Anglo-Normans, "Old English" families came to Ireland, and often, like the Anglo-Normans, tended to adopt Gaelic culture. During this period, the English crown made periodic attempts to discourage the "Gaelicization" of the recent settlers. One such attempt was the enactment of the famous Statute of Kilkenny (1366). Nonetheless, until the seventeenth century, the Crown enjoyed only limited success in its efforts to force various aspects of English culture on the inhabitants of the Pale, the area centering on Dublin that for several centuries marked the limits of English political control.

The Plantation of Ulster in 1609 marked the beginning of successful attempts to anglicize Ireland. Under rights granted them by James I, several of London's merchant companies recruited English citizens and English-speaking Lowland Scots for settlement in Ulster. William III's victories against the Catholic Irish and their allies at the end of the century secured English rule over the entire island and sealed the fate of Gaelic culture for the next two centuries.

English rule brought pressure to conform to English ways. Social and economic advancement, many Irish concluded, would come a lot easier if they jettisoned relics of Ireland's Gaelic past. One of the major consequences of this pragmatism was a tendency, during the eighteenth and nineteenth centuries, for Irish families to drop the "O" or "Mac" that might be prefixed to their surname.

With English rule also came English bureaucracy. Clerks, who were required to record Irish names for tacation or other purposes, often replaced the names with their English equivalents, or simply with English names that sounded similar. To give just three examples among hundreds, O Mordha was often changed to Moore, Cullane (O Coilcain) became Collins, and at least some O'Hagans had their names changed to Hog, which was easily taken for a variant spelling of Hogg, an English surname.

Ever since, a large percentage of the Irish people has had its origins thrown into confusion. Did a family named Collins, for example, descend from Cullanes who had had their Irish name changed to Collins? Or were their forebears Englishmen who had traditionally borne the name and perpetuated it unchanged after settling in Ireland? The case is similar for Moores, Hogs, Smiths, and countless other families.

As the British grip on Ireland began to loosen in the late 1800s, interest in traditional Irish culture grew among much of Ireland's population. Many surnames now went through yet another transformation. The trend, which has continued to the present day, was marked by widespread adoption of more traditional (or traditional-sounding) surnames.

Many Dohertys, for example, resumed calling themselves O'Doherty--or even more Gaelicized forms, such as O Dherty or O Dochartaigh. Smiths who believed that their original ancestor of the name had had his surname changed from an Irish form--or Smiths who simply felt themselves to be essentially Irish--abandoned the English name in favor of Mac Gowan, McGowan, Gowan or some other more Irish form.

For the person trying to trace his Irish genealogy, the changes that have affected many surnames over the centuries can be a major stumbling-block. At the very least, it should cause such root-hunters to proceed with great care in drawing conclusions about their family trees.