Hi Liz,
It does not surprise me that they may have been born in Ireland. I do have the census info and Andrew who I have a fair amount of info on was born in Ireland as from the census were Charles and John..
An Andrew LYNCHEY married Jane Irvine at Ballaugh on 15/06/1841 with the permission of his guadrdian William Quayle, I suspect he was born C1821.
The reference in Ancestry i think was posted by another descendent in Sydney who has bee excahnging info with me.
Thomas was the son who was tried and convicted with Sarah LYNCHEY and he was transported separately to OZZ. He had significant tattoos listed on the indent of convicts on the Mary Ann including a final reference to "...brother James." I suspect that James may have been the one son who came to OZZ with Sarah and her two daughters.
One daughter was Mary Ann who married another convict (George DUDLEY from London), they were my GG grandparents. The other daughter may have been the Sarah daughter of Michael LYNCHY and Sarah MULLEN (I have now seen the film and her name is recorded as MULLAN.
My reasons for seeking the death certificates or other records of the deaths of John, Charles and Andrew is to try to confirm the name of Sarah's husband and possibly lead me on to thier parents and further.
Some children could well have gone to North America, I do know that some of Andrew's children went to Liverpool and other parts of England as did a Charles LYNCHEY
All the references to the convict Sarah that give ages indicate C1785 so marriage during the first decade of the nineteenth century would not be unfeasable so births during the second decade are quite likely
I hope that this is not information overload.

Cheers

Peter