Brennan is an Irish surname, Ó Braonáin, from the Gaelic braon, which is usually read as "teardrop" or "sorrow", though some give it as "moisture" or "rain". It has the polished sound of Brendan without being as common. In the United States Brennan ranked number 276 in 2000 and peaked at number 242 in 2009; it has fallen steadily since, from number 444 in 2015 to number 1003 in 2025.