Irish citizens who wish to marry abroad (not necessarily in Australia) must receive a certificate of freedom to marry from the Consulate.  This certificate is known as a “certificat de coutume”. You should check with the authorities in the country in which you intend to get married, and they will inform you if such a certificate is required. The Australian authorities do not require this document for marriages within Australia.
The relevant forms can be downloaded from this website. Please see “Related Documents” below. 
It is extremely important that all forms are either type-written or completed in block-capitals to ensure that the information provided is legible. 
Applications should be made four months before the marriage is due to take place. The certificat de coutume is valid for 120 days from the date of issue.
All documents in languages other than English should be translated.

Documents required
In all cases, the following documents are required for each Irish person:  
Completed MP1 Form (Questionnaire to be completed by Irish citizen seeking a “Certificate de Coutume” for marriage abroad)
Completed MP2A form (Statutory Declaration by Irish citizen seeking a “Certificate de Coutume” for marriage abroad)
Original long form birth certificate
Copy of Irish passport
Fee of $30.00 (+ $4.00 postage if you wish for the documents to be returned to you by registered mail)
If an Irish citizen who wishes to marry abroad has previously been divorced they should also provide the following:
Completed MP2B form (Statutory declaration to be made by a divorced Irish citizen requiring a “Certificate de Coutume” for marriage abroad). This is instead of the MP2A form.
Original petition of divorce
Original Final decree of divorce
The statutory declaration form must be completed in person either at the Embassy or before a notary public.  If you wish to have your form witnessed at the Consulate, there is an additional $30 fee for this service.  If you choose to complete the form with a Justice of the Peace/Notary public you can complete the rest of the process by mail.