The Clan/Sept History

There were four septs of the Ó Braonáin clan in the country, located in Kilkenny, Galway, Westmeath and Kerry. There was also a sept of Mac Branáin in Roscommon, and Ó Branáin in counties Fermanagh and Monaghan, whose name was also Anglicised to Brennan. Brennan is now one of the thirty most common surnames in Ireland, and is most numerous in counties Kilkenny in the province of Leinster, and Sligo in the province of Connacht.

Many variations of the name O Brennan have evolved since the time of its initial
creation. In Gaelic it appeared as "O Braondin," from the word "braon," which
has several meanings, possibly meaning "sorrow" in this case.

The recording of names in Ireland during the Middle Ages was an inconsistent
endeavor at best. Since the general population did not know how to read or
write, they could only specify how their names should be recorded orally.
Research into the name O Brennan revealed spelling variations, including
Brennan, McBrennan, Brannon, Brannan, Brannen, Brannin, Brennyn, Brannyn,
MacBrennan, Brenan, Branon, Branan, Branen and many more.
First found in county Kilkenny, where they were seated from very ancient times,
long before the Norman Conquest in 1066.
A great mass of Ireland's native population left the island in the 19th century,
seeking relief from various forms of social, religious, and economic
discrimination. This Irish exodus was primarily to North America. If the
migrants survived the long ocean journey, many unfortunately would find more
discrimination in the colonies of British North America and the fledgling United
States of America. These newly arrived Irish were, however, wanted as a cheap
source of labor for the many large agricultural and industrial projects that
were essential to the development of what would become two of the wealthiest
nations in the western world. Early immigration and passenger lists indicate
many people bearing the O Brennan name: James Brennan, a bonded passenger, who
arrived in Potomac in 1731;Timothy Branen who settled in Placentia,
Newfoundland, in 1744; Laurence Brennan, on record as a laborer in St. John's
Newfoundland in 1779.