akways happy to help, you are welcome to message me. I am a Kelly by birth so if you want to connect with Kelly's around the world and find out your ancestry, please just ask x
I don't know Mary but your question intrigued me. For her to me sent to Australia and her sister to the US I would gently suggest you look at the Irish orphan scheme. It had different names but I believe it was the earl grey scheme in Australia. Famine orphans is another sad reference to these young girls. Not sure if you are in Australia but this being an ancestor I am assuming yes.
clan Kelly on Facebook could be a great connection, they are only Kelly blood and around the world so it's likely you will find the sisters family too.
there is a group but the name I can't remember but google irish famine orphans and you will find links. Mountbellew comes to kind. These girls were young, poor and in workhouses so you may find records and that way too. My first research would be finding her arrival record. It should be with nsw because it is before we were a country.