There are different conditions depends on the chosen solution, please see below our comments to both of them.
Virgin Australia/Etihad Airways
We have managed the flights to various cities in Australia (Perth, Melbourne, Sydney, Brisbane) mainly within May 2015. You may also consider to visit Tasmania for very attractive fare considering the destination. One Way tickets between Dublin and Germany should be matter of less than €25 if you book in advance as they are well connected with Ryanair.
Booking samples
Dublin - Melbourne - Germany (Frankfurt/Munich) for €664
Dublin - Melbourne and Sydney - Germany (Frankfurt/Munich) for €668
Tasmania
Dublin - Hobart/Launceston - Germany (Frankfurt/Munich) for €724
Apart of these deals you may also consider to fly with Etihad Airways/Virgin Australia and visit incredible isles in Pacific - Samoa/Fiji/Tonga from €753 or Cook Islands/Vanuatu from €770!
Royal Brunei Airlines
To book the cheapest flights to Melbourne you have to look for travel dates within September till November 2014 and in May 2015. Other months are also available but for a bit higher fare.
Booking sample
London - Melbourne for Ł603
Booking sample of cheap open jaw flights to Australia from Europe for €664 here. In this case you would be departing from Dublin to Melbourne on 5th of May. Your return flights from Australia back to Germany (Frankfurt) would take place on 26th of May 2015.