Dear Tamzine:
Many thanks for your post to the Ireland Reaching Out message board. Welcome to the website!
I have had a quick look through the Irish Genealogy website and the Roots Ireland database for you without any reference to a David Holly with those parents listed and in that time period.
There were Hollys located in Fermoy but they were not the names that you have listed and the children were baptised between 1842-1860.
Do you know when David Holly went to Australia? That might help to narrow down the search.
If you have further information, please post it.
All the best,
Jane.