Irish Citizenship
Irish citizenship follows a strict generational order: transmission to children is automatic, but eligibility for grandchildren and great-grandchildren depends on prior registration by the preceding generation.
Automatic transmission. If at least one of your parents was born in Ireland, you are an Irish citizen with no additional prior registration required.
Grandchildren of Irish citizens may reclaim citizenship following registration of their birth with the relevant authorities. This registration will in turn enable the next generation to obtain citizenship as well.
This pathway is only viable if the intermediate generation (your parent or grandparent) was registered prior to your birth. If that generation was never registered, the chain is broken and it is not possible to claim citizenship directly through the great-grandparent.
NO: Your great-grandparent was born in Ireland, but neither your grandfather nor your father ever registered. Because your father was not an Irish citizen at the time of your birth, the chain was broken.