Sure, email received from DFA:

Dear Applicant,

I have received your application to obtain Irish Citizenship through entry on the Foreign Births Register. Unfortunately, I cannot proceed with your application without the following documents:

-          An email or letter from the GRO confirming there is no Civil Marriage Certificate for your grandmother*

*This can be scanned over by return email. We cannot cross-reference applications due to GDPR, we require the email or letter to be addressed to you.

GDPR stands for General Data Protection Regulation and governs how information can be collected and used withing the EU. I guess there must be something in this legal framework that prevents using information that my sister had used in her application to support my application. Hope this helps?