The Topographical Poems of O Dubhagain and O Huidhrin (14th-15th c.)

From this list it is evident that in the formation of surnames at this period, the several families adopted the names of their fathers with the prefix Mac, or of their grandfathers, or more remote ancestors, with the prefix O’. The O’Neills of Ulster took their surname from Niall Glundubh, Monarch of Ireland, who was killed by the Danes in the year 919; the O’Briens of Thomond took theirs from Brian Borumha, Monarch of Ireland, who was slain at the battle of Clontarf in the year 1014; and it will be seen that the ancestors of the most distinguished Irish families, whose names have been preserved in the surnames of their descendants, flourished from the year 900 to 950, or 1000. A few exceptions will, however, be found, as in the family of O’Dowda of Tireragh, whose progenitor, Dubhda, flourished about the year 876; in that of O’Kelly of Ui-Maine, whose ancestor, Ceallach, flourished as early as the year 874; and in that of O’Ruarc, of Breifny, whose progenitor, Ruarc, flourished from about the year 820 until 893.

There are also instances to be met with of surnames, established in the tenth century, having been changed to others which were taken from ancestors who flourished at a considerably later period, such as O’Mulrony of Moylurg, who assumed the surname of Mac Dermott from Dermott, chief of Moylurg, who died in the year 1159; The year 1159.—Memoirs of Charles O’Conor, page 305. and O’h-Eochy of Ulidia, who changed the family name to Mac Donlevy.

There are also instances of minor branches of great tribes having changed the original prefix O’ to Mac, or Mac O’, or I, when, having acquired new territories for themselves, they became independent or separate families. Examples include O’Brien changing to Mac I-Brien and Mac Brien, in the instances of Mac I-Brien Ara, Mac Brien Coonagh, and Mac Brien Aharlagh, all offshoots from the great O’Brien family of Thomond; and O’Neill to Mac I-Neill Buidhe, in the instance of a branch of the Tyrone family who settled in the fourteenth century in the counties of Down and Antrim. These surnames having been rejected in modern times, the original surnames of O’Brien and O’Neill have been restored.