It derives from three Irish originals: O Braonain, from braon, possibly meaning "sorrow", Mac Branain, and O Branain, both from bran, meaning "raven", one of the most popular personal names in early Ireland.
The Mac Branain were chiefs of a large territory in the east of the present Co. Roscommon, and a large majority of the Brennans of north Connacht, counties Mayo, Sligo and Roscommon, descend from them.
O Branain was a name found among the Cenel Eoghan, the tribal grouping claiming descent from Eoghan, one of the sons of the 4th century monarch, Niall, progenitor of the Ui Neill. Their territory included the modern county which records his name, Tyrone, and it is in this area and the adjoining counties of Donegal, Fermanagh and Monaghan that the northern Brennans are now most commonly found. In this area, especially Donegal, the name was also anglicised as Brannan and Branny. 15 Brannan births are recorded in 1890, most of them in Donegal.
O Braonain originated in at least four distinct areas, Kilkenny, east Galway, Westmeath and Kerry. Of these the most powerful were the O Braonain of Kilkenny, chiefs of Idough in the north of the county. After they lost their land and status to the English, many of them became notorious as leaders of bands of outlaws. Witness the popular ballad "Brennan on the Moor".
One of the last chiefs of the name of the .Kilkenny Brennans was John Brennan (1768-11830), popularly known as the "wrestling doctor" for his satires on the Dublin medical establishment.
The classical scholar Christopher (John) Brennan was the most learned poet Australia produced at the end of the 19th and the beginning of the 20th century., Mostly in the symbolist tradition, his work is characterized by force of feeling and depth of imagery.
Joseph Brennan (1887-1963) was one of the most influential Irish civil servants of his generation, becoming the first Secretary of the Department of Finance, chairman of the Currency Commission and, from 1942 until 1953, first governor of the central Bank.