You will need to pay for professional advice from someone who understands the interplay of Irish, Australian and UK tax and pensions but here are some general principles.
If you are planning to settle in Ireland, it makes little sense to have a property in Australia. You will face complicated income tax, CGT, and CAT issues which could be expensive to resolve.
If you have around €300k when you are buying a house for €600k, you will get the lowest available mortgage rates - currently around 3.6%. If you borrow 80% Loan to Value, you will probably pay around 4%. So it's well worth borrowing as little as possible.
You are buying a long term family home. You can afford to pay more than €600k, so buy the home you want to live in, within reason.
So sell your property in Australia.
Brendan