The basic requirement for obtaining Irish citizenship by descent, through Foreign Births Registration is that an applicant's parent was an Irish citizen at the time of the applicants birth.
A person born outside of Ireland with a parent who was born in Ireland is automatically an Irish citizen by descent. A person born outside of Ireland with parents who were born outside Ireland and a grandparent who was born in Ireland may become an Irish citizen by Foreign Births Registration.
If a person wishes to apply for Irish citizenship through a great grandparent born in Ireland, the position is more complex. Eligibility depends on one of the applicant parents, i.e. the grandchild of the person born in Ireland, being already entered in the Foreign Births Register, the date of that registration and the applicants date of birth.
Under the 1956 Act, an applicant who was entered in the Foreign Births Register between 17 July 1956 and 30 June 1986 acquired Irish citizenship from the date that the Act came into force (17 July 1956) or the applicants date of birth, whichever was later. Under the 1986 Act, a person is accorded Irish citizenship from the actual date of entry in the Foreign Births Register.