You declare the gross amount of dividends in your Irish Tax return plus the amount of franking credit, assuming it is not in excess of 15%
If it is in excess of 15%, then you can only declare the 15% as a tax credit on your irish tax return and must reclaim the amount above 15% back from the Australian tax authority. The Double taxation treaty limits the amount of tax on dividends to 15% - unless your a company or related party
Depending on your overall tax position in Ireland you may get credit for some or all of the tax deducted by the Aussies