Crack on

I knew a man who used the phrase 'crack on' as a synonym for 'you must be kidding', or 'get real'. Has anybody else come across this, and do you think it might be related to the Irish 'craic'?

I haven't come across this, but your suggested derivation sounds unlikely to me. "Crack on" means "to progress briskly" (cf. "get cracking"), and as such its usage to mean "you must be kidding" is precisely analogous to a whole raft of phrases like "Get along with you!" "Get away!" "Garn(i.e. "Go on")!(VSD)