I wonder if this is Arthur Jnr, same area as Nicholas?
I have no idea why the registration numbers are the same.

1864 ONEIL Margaret parents Nicholas ONEIL and Maria DENANDS Strathalbyn 30/667
1864 ONEIL Margaret parents Arthur ONEIL and Hester CRITCHLEY Strathalbyn 30/667
1861 45/89 ONEILL Arthur to CRITCHLEY Esther Mount Barker
Brother James? 1861 48/106 ONEILL James to MCCOURT Martha Mount Barker