Brennan is an Irish surname name — anglicized from the Gaelic Ó Braonáin, meaning "descendant of Braonán," where braon means "moisture" or "drop of water" — that peaked in 2009 with 33,032 SSA records. Now at #1054, Brennan is post-peak but still carries the warm Irish-American authenticity that made it a staple of the Celtic name wave.
Irish Gaelic Roots
The Gaelic braonán is a diminutive of braon (moisture, teardrop, raindrop) — a poetic, water-based etymology that has its own quiet beauty. The O'Brennan surname is one of the oldest in Leinster, with strong roots in County Kilkenny. As a given name, Brennan entered American use through Irish-American families who converted their surnames into first names — a practice that produced Brady, Casey, Callahan, and Connelly. Irish surname-first-names with this kind of authentic clan history carry a genealogical weight that invented names simply don't have.
The 2009 Peak and the Irish Wave
Brennan peaked in 2009 at the tail end of a decades-long Irish name renaissance in America. Conor, Declan, Liam, and Sean had been building since the 1990s; Brennan represented a slightly less common choice for parents who wanted something recognizably Celtic but off the obvious track. The 33,032 total records confirm it achieved real mainstream use during that window. The 2000s were the peak decade for the second wave of Irish names in American use.
Counter-Reading: The Post-Wave Position
Brennan has declined since 2009, which places it in the awkward middle territory, not quite vintage enough to feel retro, not current enough to feel fresh. In about fifteen years it may start reading as nostalgic-cool. For now, it's a solid, handsome name that a certain kind of Irish-heritage parent will love without ambivalence. Compare with Brendan and Declan on the rankings page for the full Irish field.