I looked it up for you and this seems to be official government info:
Citizenship through descent from Irish grandparent If one of your grandparents was born in Ireland, but neither of your parents was born in Ireland, you may become an Irish citizen. You will need to have your birth registered in the Foreign Births Register.
Here is more on Foreign Births Register:
https://www.citizensinformation.ie/en/moving-country/irish-citizenship/foreign-births-register/
Looks like it can also go one step further for great grand parents. I'm going to try and call in the next few days to the Irish foreign office and see what they say.
Good luck, hope it goes well.
(post is archived)