Quick answer
Brennan is a boy name.
Meaning and history
From an Irish surname (Anglicized from Irish Gaelic Ó Braonáin ) that was derived from the byname Braonán, itself from Irish braon meaning "rain, moisture, drop" combined with a diminutive suffix. As a given name, it has been used since the 1960s as an alternative to Brendan or Brandon, though it has not been as popular as them.
Notes and context
Usage: english