Why must my parent be registered before I am born to pass on Irish citizenship?

Irish citizenship by descent depends on the parent being an Irish citizen at the moment the child is born. If a parent registers in the Foreign Births Register only after a child is already born, that child cannot claim through them, because the parent was not yet a citizen at the relevant time.

Children born after a parent's registration are eligible to apply; children born before it are not. This timing rule is the single most important point for families to understand when planning across generations.