Very silly to take a risk of signing a contract if he were to pull out and he in Australia and you to be liable!
Yes you can bid for him. But you need a deposit cheque from your brother. Is this at Allsops auction? They can accommodate this. He can even pay them the depsoit now and bid himself over the internet or on the telephone.
Why is your brother in Australia buying a house in Ireland ? Because it's cheap, it's an investment, he will return, property prices will rise.
How is he able to fund the purchase.