Brennan is one of the 30 most common names in
modern Ireland. It originated as the name of several unrelated septs.
Most of these seem to have originally used the Irish form UaBhrainain
or UaBraonain, which was later anglicized as O'Brennan and later
The Kilkenny UaBraonains, descendants of Braon,
(sadness or sorrow), were chiefs of "Ui Duach (Idough) in the
barony of Fassadinin and the lowlands of Glamoy and Crannagh
adjoining the Noir. They were prominent at the end of the Middle Ages
and, of all the Brennan families, these Kilkenny O'Brennans (or
Brennan of Ossory) seem to have survived in the largest numbers and
account for the ancestry of the majority of Brennans today.
The MacBranain of County Roscommon were the
second most populous of the various septs, accounting for over a
quarter of all the Brennans in Ireland. Unlike the other septs, they
were not really O'Brennans at all. Alone among all the septs, they
originally used the "Mac" instead of the "O."
(Mac means son of. Branain is the diminutive for "Raven").
The spelling of the name varies in the annals between MacBranan and
MacBranain. According to MacLysaght, the great authority on these
things, the present day Brennans of Roscommon, Sligo, Mayo and
Leitrim are nearly all originally MacBranans. The majority of the
Brennans now living in Galway probably also originated as MacBranans
The UaBraonain of Crevagh in County Westmeath was
once a powerful clan. They were largely displaced after the Norman
invasion of the twelfth century. The name is still common in the
county of Westmeath today, though in the west of the county, many may
be of MacBrennan of Roscommon stock.