Brannan is an uncommon Irish surname derived from "Ó Branáin," meaning "descendant of Branán," who was named after the Old Celtic word for "raven." It originated in medieval Ireland and was prominent among the Ó Branáin clan in County Mayo, known for their bravery. The name has been associated with notable figures like American Civil War general Thomas W. Sherman, contributing to its global spread. Today, Brannan is gaining popularity as a given name due to its unique sound and Irish heritage.