In modern Ireland there are many Brennans: the name comes twenty-eighth in the statistical list of Irish surnames. Here and there one is met with the prefix O, but to-day the form MacBrennan is seldom if ever found.

The simple form Brennan is used in the anglicized form of two quite distinct Gaelic Irish surnames, viz. O Braonain and Mac Branain. The former is the appellation of four different unrelated septs; the latter of one only. Judging by the present day distribution of the name, two of these five have survived in large numbers in the districts around their original habitats. It is sufficient, therefore, just to mention en passant the three others which were located respectively in counties Galway, Westmeath and Kerry.

Mac Branain was chief of Corcachlann, the old name of a territory in the eastern part of Co. Roscommon: a succession of these chiefs appear in the Annals between 1159 and 1488. While the leading members of the sept retained the Mac until the submergence of the Gaelic order in the seventeenth century, the substitution of O for Mac, in some cases, is noted as early as 1360. The present day Brennans of Counties Roscommon, Sligo and Mayo, however, are nearly all MacBrennans, or more correctly MacBrennans.

The principal O'Brennan sept was that of Ossory: they were chiefs of Ui Duach (mod. Idough) in the northern part of Co. Kilkenny. Their influence naturally waned as English power became paramount in Leinster, and though several O'Brennans retained some portion of their former estates, the seventeenth century reduced many of them to the status of rapparee - indeed, several famous or notorious bands of tories in Leinster were led by Brennans, and in the next century, one of the most intrepid and chivalrous of all Irish highwaymen, James Freney, was, he asserted, instructed in his calling by the last of these tory Brennans.

The most distinguished of the sept was Most Rev. John Brennan (1625-1693), Bishop of Waterford and Archbishop of Cashel, friend of Geoffrey Keating and Saint Oliver Plunkett: though constantly the object of special attention from priest-hunters, he was elusive enough to remain continuously in his dioceses which he administered with marked wisdom, and his periodical reports to Rome are of the greatest value to the historian of the seventeenth century.

Another John Brennan (1768-1830), popularly called the "wrestling doctor" and well known in his day for his satires on Dublin doctors, was also of the Ossory sept of O'Brennan and considered to be chief of the name. Among exiles of the name we may mention the Abbe Peter O'Brennan who was executed in 1794 for his resistance to the French Revolution.

The Ancient O'Brennan's - The following is a description of the O'Brennan's of Ui-Duach as appears in the Journal of the Royal Society of Antiquaries of Ireland, Volume 5:

The Upper Valley of the Nore, or Airgad-Ros, did not form part of the Kingdom of Osraighe for some period after its original establishment. In the "Will of Catheir More," a document supporting to be as old as the second century, "Airgad-Ros" is named as being subject to that King of Leinster; and it seems to have been wrested from the dominion of his successors by Duach, from whom it first asumed the name of Ui-Duach, or the land of Duach, now Odogh. This Duach was King or Chief of Osraighe about the middle of the sixth century. In the "Annals of the Four Masters" and in the "Will of Catheir Mor" this district is invarably recognized as "Argnd-Ros" down to th death of Duach, which happened before the year 580. The first entry after his death referring to the place in the Irish Annuals is at the year 850, when it is named "Ui-Duach in Argad Rois," and in all subsequent notices Ui-Duach is adopted as its recognised title. The prefix "Ui" is to be understood as governing word with which it is compounded in the genitive case, the word country or land being understood. Hence Ui-Duach Idagh, or Odagh, implies the land or country of Duach. It seems highly probably that this Duach erected his fort or earthen palace at Three Castles, anciently called "Castle Duach," which is the site also of the ancient church of Odagh; and in the garden of "Three Castles demesne" is a high sepulchral mound, now planted with shrubs and flowers, and to the summit of which you ascend by a curious spiral terrace; it is within a short space of the ancient graveyard, and seems highly probable to have been raised over the body of this chief lord of Osraigh, whose name the place assumed, and still adopts as its own. In O'Heerin's "Topograph" we have the following reference to the country of Ui-Duach:

"Ui-Duach of Ossory of the warm soil,
The fair wide plain of the Feoir.
Not easily passable is the wood of the plain;
Its protecting chieftain is O'Broenain."

Ui-Duach is identified with the bold adventurs and daring exploits of this valiant tribe of the O'Breannain, son of Cearbhall Mac Dunghal, King of Osraigh, in the ninth century. It does not appear that the O'Breannains at any time held possession of the entire territory of Ui-Duach. Their dominion seems to have been confined to the Fassach or waste of the Dinan River, including the fertile spots and rich pasture lands lying between the hills of Fassidineen. The Comar was the head-quarters of capital bailli of this celebrated tribe land. For the history of this turbulent but valiant tribe, who held their own in this secluded region despite English power, down to a comparatively modern period, seee "Tribes and Territories of Ancient Ossory," by the Rev. James Graves.