Have you done any checking in Ireland? So identifying siblings and then tracking them forward.
The Irish had a naming pattern and this can be useful in tracking the families.
he Irish naming pattern is as follows:
1st son is named after the father’s father.
2nd son is named after the mother’s father.
3rd son is named after the father.
4th son is named after the father’s eldest brother.
1st daughter is named after the mother’s mother.
2nd daughter is named after the father’s mother.
3rd daughter is named after the mother.
4th daughter is named after the mother’s eldest sister.
What were the names and birth order of the children of this couple.
I agree it is possible that siblings may have already arrived but do not overlook less closely related relatives and also that they may have gone to different parts of NZ. I had relatives who I think were part of a wider family chain migration - cousins from the same village and they went to Timaru and my lot stayed in Auckland.
This is a good website
https://www.irishgenealogy.ie/en/ETA: I have seen some records are under the name of O'Sullivan and that Catherine Hanlon's name is some times recorded as Katherine.