3Killanin, Michael Morris, and Michael V. Duignan. The Shell Guide to Ireland. London: Ebury P. in Association with George Rainbird, 1967. 166.
According to this online forum, Mhaistin may also derive from "Mastiff," or "unruly child," or "large ugly thing." Some claim that the name Rostellan comes from Ros (headland) and dallan or dolmen.