More can be always said but look for answers by u/koredozo and u/Zither13 presented here as well as ones by u/mxworthing and u/litlnemo presented here They agree with each other on multiple possible sources for the surname King: ironic nickname, mistranslations that occurred when Irish surnames were Anglicized as well as changes in spelling affecting other words, and pageant names (surname given to one who played a biblical King during a mystery play).