Brennan is an Irish surname which is an anglicised form of two different Irish-language surnames: Ó Braonáin and Ó Branáin (or Mac Branáin). Historically, one source of the surname was the prominent clan Ua Braonáin (O'Brennan) of Uí Duach (Idough) in Osraige who were a junior Dál Birn sept stemming from a younger son of Cerball mac Dúnlainge (d.888). Recent surname evaluations highlighted the geographic consistency of this lineage in the barony of Idough. However, based on the ultimate authority of Dubhaltach Mac Fhirbhisigh they are out of Ui Dhuinn (O’Dunn) and, therefore, an Uí Failghi tribe, not Osraige. While it is clearly apparent that O’Hart's pedigree is erroneous, it is suggested that Ó Cléirigh probably became confused while transcribing from Mac Fhirbhisigh. This is echoed by the modern scholar, Bart Jaski. The Irish surname Ó Braonáin, means "descendant of Braonán". The personal name Braonán is derived from a word which means "moisture", "drop". The Irish surname Ó Branáin, means "descendant of Branán". The personal name Branán means "little raven". Brennan is also a given name. Notable people with the surname include:

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