According to the Cambridge dictionary it is a remarkable or astounding person or thing. The history of corker is really about bottle corks and not about Irish from Cork. It means a settler, as in a thing which settles (a debate, wager, etc.)

Probably an agent noun from cork (v.) on the notion is of putting a cork in a bottle as an act of finality

Funny side to the meaning is to tell a joke or story such as…. She told an absolute corker of a story about a priest she'd mistaken for an ex-lover.

In the 1830s it was used as a verb to mean "something that settled a debate." The 1830s sense is probably from the verb "to cork" as in the stage of making a bottle of wine that "finalised" the act. By the 1880s, it had morphed to mean "something astonishing."

Australia's local lingo. ... Corker - something striking or astonishing; something very good of its kind.