Just some information for anybody that's wondering. Estimates seen to vary but it looks like the following applies to 5 or 6 million people.
If you were born outside the island of Ireland and if either of your parents was an Irish citizen who was born in the island of Ireland, then you are entitled to Irish citizenship, and entitled to apply for an Irish passport under Irish law, irrespective of where you reside. You can also apply for Irish citizenship if one of your parents, while not born in the island of Ireland, was an Irish citizen at the time of your birth. In most cases applications of this type are made through a grandparent who was born in Ireland.
www.dfa.ie/irelanduk-citizenshipandpassports/