I've never heard that term being used as such.
There is another Rocksavage on the coast near Clonakilty, I would be thinking someone who had connections to that locality, a country retreat perhaps, named the city area after it.
Alternatively there were Earls of Rocksavage (A courtesy title of Marquesses of Cholmondeley) so may be named to honour or curry favour with them, and/or an attempt to put a respectable veneer on the area perhaps.