Ireland - Passport & Nationality - Irish Citizenship by Double Descent
If you were born outside Ireland to a parent who was also born outside Ireland, and one of your grandparents was born in Ireland (which includes Northern Ireland for the purposes of this solution), you are entitled to register as an Irish citizen. However, before you can claim Irish citizenship, you must have your birth registered in the Irish Foreign Births Register.
Watch this SHORT VIDEO by Philip Gamble on how Irish nationality is gained in this manner:
Irish citizenship can be passed down through successive generations, provided each generation is registered in the Foreign Births Register before the birth of the next generation. Children born before their parent's registration in the Foreign Births Register are not entitled to Irish citizenship through that parent. Unless at least one parent or grandparent was an Irish citizen at the time of your birth, you cannot claim Irish citizenship on the basis of extended previous ancestry or wider family ties, such as the Irish citizenship of cousins, aunts or uncles.