I was looking at a transcript of the original ships passenger list, and wondered if you have the full information. It is as follows:
OMEGA departed Plymouth 13th Aug 1857, arrived Pt Adelaide Sat 14th Nov 1857. On board were 114 English, 18 Scots and 178 Irish. The Miller family was...
James Miller age 33 Carpenter Dumbarton.
Mary Ann Infant
O'Neil? Ann 14 Niece? .....with this family.
All these christian names are very common, so that makes it difficult, especially since there were no Electoral Rolls at that time. Only chance to discover what happened to Ann O'Neill would be to find a marriage between say 1862 [when she would be 19] and at a guess
1873 when she would be 30.