Fair dinkum is not an Irwinism. It goes back to the 1800s I am sure. It means, "for sure" or "no doubt". Or in its inquisitive form, "Fair dinkum, eh?" meaning "you don't say".
How it came about, well, FIIK.
BUT ....
"Fair dinkum, mate, the Aussies are gunna whip the pommies' arses today in the cricket".
Regards