I can confirm that the Department of Foreign Affairs can issue a civil letter of freedom (Certificat de Coutume) to Irish citizens getting married abroad. This is not a requirement under Irish law, but is sometimes required by the relevant authorities in the country in which a citizen wishes to get married.
The Certificat de Coutume that the Department of Foreign Affairs issues is a standard certificate which contains the applicant's name, date of birth, passport number, and address, and which states that the applicant is free to marry. No other information can be added to the Certificat de Coutume.
In the case of certain Member States of the European Union, we provide, along with the Certificat de Coutume, a Multilingual Standard Form (MSF) which is a translation of the information provided on our Certificat de Coutume, and is used throughout the EU.
In this case, Certificats de Coutume were issued for the individuals in May. I can confirm that our Embassy in Athens has already been in further contact with the individuals, and that the issue regarding recognition has been addressed.