Hi Robin - You didn't say what place of birth was given for Robert in the 1851 census you mentioned? Did any of his records have anything about him other than his age?

The original passenger list of the Dorset should have recorded where Mary Ann came from. There is a partial passenger list on the internet of the Dorset arriving Port Adelaide Dec 11 1847 which includes a Mary Ann CORHL with 3 male children and 3 female children - is this the date?

There was a CORKHILL Mary Miss arriving in Sydney on the Cronkbane on 31/7/1853 from Melbourne (you didn't give her marriage date in 1853).
And there was a CORKHILL MARY aged 19 arrived Victoria JUL 1853 on the 'IDA' Book 9 p.198, as an assisted immigrant (full record at the PRO Vic.)

The discrepancy in the burial/death record could have been a transcription error, with a 1 looking like a 7, or vice versa, unless you were looking at the original or photocopies of the records.

Sue