O'Herlihy was born in Dublin, Ireland, the son of Elsa Bennett and Irish actor Dan O'Herlihy. In his youth, he was an avid tennis player, and even became Irish National Tennis Champion. He has over thirty screen credits to his name, most of them having cast him in villainous or antagonistic roles such as Death Wish 3, Never Say Never Again, and Superman III. His role as Airk Thaughbaer in the 1988 fantasy Willow is one of the few heroic roles the burly, fair-haired Gavan has managed to get, as well as the dashing American officer Captain Leroy in Sharpe's Eagle.
He was also the jock son Chuck Cunningham on Happy Days, but it wasn't a very big part and he was eventually written out by being sent to college and then disappeared with no mention. He didn't want to get stuck in television, preferring the film medium. Gavan appeared in the pilot episode of Star Trek: Voyager entitled Caretaker as the Kazon First Maje, Jabin.
He has a son called Rogan.