Unfortunately, it sounds like that the office may have lost your certificate. They should be returned along with all other supporting documents.
Use WebChat during working hours: https://www.dfa.ie/passports/contact/
The WebChat button will be enabled between 9am - 5pm on weekdays.
In the case that your certificate is confirmed lost by DFA, you can apply for an official statement confirming your citizenship status. You shouldn't have to do that (as you already have your passport) but it's good to have an official correspondence.
https://www.irishimmigration.ie/wp-cont ... t-Cert.pdf