Brennan

It derives from the two Irish originals O Braonain and Mac Branain. The Mac Branain were chiefs of a large territory in the east of the present County Roscommon, and a large majority of the Brennans of North Connacht, counties Mayo, Sligo and Roscommon, descend from them.

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.

A separate family, the O Brainain, are the ancestors of many of the Brennans of counties Fermanagh and Monaghan, where the name was also anglicised as Brannan and Branny.