Wikipedia for what its worth
http://en.wikipedia.org/wiki/Irish_n...y_law#At_birth
Due to legislative changes introduced in 1986, the Irish citizenship of those individuals requiring registration, dates from registration and not from birth, for births registered on or after 1 January 1987.[17] Citizenship by registration had previously been back-dated to birth.
In practice, anyone with an Irish citizen grandparent born in the island of Ireland, can easily claim Irish citizenship. His or her parent would have automatically been an Irish citizen and their own citizenship can be secured by registering themselves as in the Foreign Births Register. In contrast, those wishing to claim citizenship through an Irish citizen great-grandparent may be easily frustrated if their parents were not registered in the Foreign Births Register. Their parents can only transmit Irish citizenship to children born after they themselves were registered and not to any children born before registration.