Person ID vs Old Department of Justice reference number

Hi i'm making my application for residency and I'm stuck at the Person ID/Immigration Details section. i have registration number in the form of **/***/** which seems to indicate the year of arrival. i have a GNIB card and a PPS number.
I don't know if my registration no. is my Person ID or Old Department of Justice reference number
I'm filling it in online and can't find any guidelines whatsoever
Thanks

For residency (to renew IRP) or naturalization?

In online forms for Dublin renewal this field isn't mandatory! Just ignore it.

For naturalization, do you have any physical letters from ISD?

Actually my mistake, i'm applying for naturalization, ive been a resident since 1997 on Stamp 4, i don't have any correspondence from ISD, not even sure what that is tbh

ISD is the new name for INIS.

If you had any physical correspondence with them recently, they have person ID in the top right corner.
If you didn't, my best advice would be to send an email to either citizenshipinfo@justice.ie or immigrationsupoort@justice.ie (or both), asking if they can provide you with one.

ok will do that, thanks for the help