It appears, then, from the most authentic annals and pedigrees, that the O’Briens of Thomond took their name from the monarch Brian Boru himself, who was killed in the battle of Clontarf in the year 1014, and that family names were formed either from the names of the chieftains who fought in that battle, or from those of their sons or fathers:--thus, the O’Mahonys of Desmond are named from Mahon, the son of Kian, King of Desmond, who fought in this battle; the O’Donohoes from Donogh, whose father Donnell was the second in command over the Eugenian forces in the same battle; the O’Donovans from Donovan, whose son Cathal commanded the Hy-Cairbre in the same battle; the O’Dugans of Fermoy from Dugan, whose son Gevenagh commanded the race of the Druid Mogh Roth in the same battle; the O’Faelans or Phelans of the Desies from Faolan, whose son Mothla commanded the Desii of Munster in the same memorable battle, as were the Mac Murroghs of Leinster from Murrogh, whose son Maelmordha, King of Leinster, assisted the Danes against the Irish monarch. The Mac Carthys of Desmond are named from Carrthach (the son of Saerbhreathach), who is mentioned in the Irish annals as having fought the battle of Maelkenny, on the river Suir, in the year 1043; the O’Conors of Connaught from Conor or Concovar, who died in the year 971; the O’Molaghlins of Meath, the chiefs of the southern Hy-Niall race, from Maelseachlainn or Malachy II, monarch of Ireland, who died in the year 1022; the Magillapatricks or Fitzpatricks of Ossory from Gillapatrick, chief of Ossory, who was killed in the year 995, &c. &c. From these and other evidences furnished by the Irish annals, it appears certain then that the most distinguished surnames in Ireland were taken from the names of progenitors who flourished in the tenth or beginning of the eleventh century. But there are instances to be met with of surnames which had been established in the tenth century having been changed to others which were called after progenitors who flourished at a later period, as O’Malroni of Moylurg, to Mac Dermot, and O’Laughlin, head of the northern Hy-Niall, to Mac Laughlin. There are also instances of minor branches of great families having changed the original prefix O to Mac and Mac O, or Mac I, when they had acquired new territories and become independent families, as O’Brien to Mac I-Brien, and Mac Brien in the instances of Mac I-Brien Arra, Mac Brien Coonagh, and Mac Brien Aharlagh, all off-shoots from the great family of Thomond; and O’Neill to Mac I-Neill Boy, in the instance of the branch of the great Tyrone family who settled in the fourteenth century eastward of the river Bann, in the counties of Down and Antrim.