I don't think so, they are two different names, I've never heard anyone refer to Ken Doherty as Docherty and i know a Docherty who is never called Doherty.
Cyanide is probably right, it depends on local dialect or regional pronunciation. For instance, the golfer Padraig Harrington is variously referred to as 'Porrig', Podrig', 'Podraig' or 'Padrig'. It is obviously a derivation of Patrick and his Mum is on record as preferring the 'Padrig' version.