I believe a lot of Irelands central records were "lost". I believe there was a fire.
I was researching my Dads side and discovered my nan ( Dads mum, i never met her) was brought up in a home- she was illegitimate. I tried to get her birth cert but no joy. I know now ( through living family and their local community in Ireland) my nans mum was young and not married and nans Dad was married but no idea who he was . This was all well over a 100 yrs ago. No parents for nan and no record/cert. Lost in the fire? I then discovered nan was not married either when she had my Dad- but they kept in touch, he was in a home also as a child. I cannot find his records either ( 1930's). It is like they both never existed?
I have found there is also a wall of silence in Ireland OP around births etc from those days. Not only officially ( the fire) but within my family itself. Most remaining/surviving records in Dub Castle. You should go there to see what you can find. I personally found nothing.
Do you know where yr grandparent was born? If so, go there. There will be someone near that address that knows and has had information passed down to them. Even now when i visit my Dads half siblings in Ireland , aged in their 90's ( nan married and had 10 further children) i can see they know more than they are saying. My cousins in Ireland and me are all trying to solve this before that last link goes . They ( the elders) will not talk and parish priest is neither use nor ornament. I suspect Op , some things will always be left unanswered.
From my experience when you cannot find records in Ireland, there is something within them you were never supposed to know ! I, like you cannot trace my family without my nans birth cert but, no one seems to know where that is?. I have no names for her parents either so the investigation stops there. People who clearly never wanted to be found, have not been. It is not easy to trace family in Ireland.
Have you posted this on Craicnet? Great resources there.