A modern spin on more common Irish boys' names like
Brian or
Brandon, Brennan means either "little raven" or "descendent of the sad one," and sounds at once unusual and familiar. It's alternate, contemporary spellings include Brennin, Brennen and—of course—Brennyn. But watch out: it's increasingly chosen as a name for little girls, which means, if history is any indication, it will be used as a boy name less and less.