Naturalisation Application Form - Person ID Number

Hi all,

Sorry for asking so many questions today! Ran into an issue on the naturalisation form while my husband was filling it out. In section 2 it is asking for his Person ID Number.
I haven't a clue what this is and I'm not sure where to find it. He must have one however as he is non EU so he must have been assigned one when he applied for permission to remain and got his stamp 4 right?

I checked his old GNIB Card and his current IRP card. None of them have this number recorded. We also don't have any letters from INIS because we register in the local garda station so they do the registration and the cards are sent to the garda station where we go pick them up. We've never seen a letter from INIS before.
Does anyone know is there anyone we can call to check if he has this number? I don't want to leave it blank if he does have one and have his application rejected.
I'll have him email the immigration officer in the garda station now to ask but they could take ages to get back to him.

Keep it blank !

When he applies for a written permission from INIS, (e.g. when he sent the EUTR application), all letters, acknowledgments, approvals should have the Person ID number. If you longer have a copy of any of these letters, or if the letters do not have the number populated for whatever reason, just leave it blank on the application form.