The association of Kerr with cearr 'left-handed' is considered to be a 'legend', as you can see at
http://www.ancestry.co.uk/facts/Kerr-family-history.ashx. There is no mention of any such association in Black's The Surnames of Scotland' which is the authority on the subject of Scottish family names.
M(a)c Ginley is derived from Fionn+ghal whereas Fingal comes from Fionn+ghall, and the difference between the second roots indicates that one comes from gal 'valor' and the other from gall 'foreigner' (previously Northman, Dane). It's the same principle as English 'gal' and 'gall'.