I think lists of Newgate executions are online somewhere so the name O'Hara could perhaps be traced there. And of course as most of these highwaymen of the 17th/18th centuries operated near London there is every chance the ballads were written in London.

If O'Hara was operating in Ireland and the ballad was written there why bother with the description 'Irish'? I doubt very much if there were any highwaymen operating in Ireland who were of other nationalities.

Also I don't think too much weight should be put on the use of the word 'flying'. Presumably they were just fast movers like Swift Nick and The Flying Scotsman/Flying Dutchman.

I must fly!