Hi,
I had a look at the available records but couldn't find anything definitive on a Eugene Kenny from the Castleisland area. The Church Records for Castleisland start from around 1820 and I did find some Kenny families living in the region at that time but none of these had a Eugene. I noted on one of his transportation records that on leaving Ireland he was married with three children, but without knowing the names of his wife or any of his children, it wouldn't be possible to link him to any of the Kenny families living in Castleisland at the time. I wonder if you have a list of the names of the children born in Australia? Irish families of the time tended to follow a particular naming tradition, and this might provide some clues. I also wonder if anyone in your family has taken a DNA test, and if so, did it provide any leads to Kenneys that aren't directly descended from the Australian branch? My hunch about the Eugene Kenny that sponsored Catherine Mansfield in Dingle, is that he's unlikely to be your ancestor. There were a number of Kenny/Kennie families living in the Dingle area at the time and that Eugene is probably one of those. We would need further evidence of a link to the Mansfields for verification.
If you can give me any more information, I will see if I can be of further assistance.
Best Wishes,
Joan Holland