I have the Maughold parish register entries, but I need more clues. Corkill was a very common name in the IoM in several parishes as well as Maughold.
Didn't the certs say how long Mary Ann had been in Australia? Did you find her on a passenger list? Wasn't any other information given about her on either cert. other than her year of birth as 1821? (Did you get this from her age at marriage?)

Also, what was the place of birth given for Robert Corkill in the 1851 census?

Sue