Citizenship through grandparent

Hi,
I am looking to become an Irish citizen through my grandparents..
My mother & I were born in London, UK.

Filling in the form online now it seems to be possible.. however it's asking for a FBR Entry Number for my mother. She was never registered for Foreign Birth Register I don't believe.

So I'm not sure what to do next.. as I understand it, she should be an Irish citizen by default as both her grandparents were born and raised in Ireland..

Any assistance greatly appreciated

Thanks

Do you mean her parents or grandparents?

Sorry.. I mean her parents yes. My grandparents.

But I think I solved it.. I just leave that blank right? As it has a red asterix, I didn't think it would let me continue until I filled that in but it does let me continue without that information.

Presumably it's for people who's parents are registered and not citizens by default through Irish born parents..

Would that be correct?