some more info...
http://www.inis.gov.ie/en/INIS/Pages/WP07000162 and click on the link
Citizenship through descent
If either of your parents was an Irish citizen at the time of your birth, you are an Irish citizen, irrespective of your place of birth (unless one of the special conditions relating to birth outside Ireland applies; these are described below). If the parent through whom you derive Irish citizenship was not alive at the time of your birth, but would have been an Irish citizen if alive at that time, you are also an Irish citizen. You derive citizenship through an Irish parent whether or not your parents were married to each other at the time of your birth.
If you were born in Ireland to an Irish citizen parent, then you are an Irish citizen by descent and also entitled to be an Irish citizen because you were born in Ireland (see Citizenship through birth in Ireland above).
If you were born outside Ireland to an Irish citizen who was himself or herself born in Ireland, then you are an Irish citizen.
If you were born outside Ireland to an Irish citizen who was himself or herself born outside Ireland, and any of your grandparents was born in Ireland, then you are entitled to become an Irish citizen, and can do so by having your birth registered in the Foreign Births Register maintained by the Irish Department of Foreign Affairs. You can do this by applying to your nearest Irish embassy or consular office. A list of these is available on the website of the Department of Foreign Affairs at
www.irlgov.ie/iveagh. If you are entitled to register, your Irish citizenship is effective from the date of registration.
If you are of the third or subsequent generation born abroad to an Irish citizen (in other words, one of your parents is an Irish citizen but none of your parents or grandparents was born in Ireland), you may be entitled to become an Irish citizen by having your birth registered in the Foreign Births Register; this depends on whether the parent through whom you derive Irish citizenship had himself or herself become an Irish citizen by being registered in the Foreign Births Register before you were born. If you are entitled to register, your Irish citizenship is effective from the date of registration. The Irish citizenship of successive generations may be maintained in this way by each generation ensuring registration in the Foreign Births Register before the birth of the next generation.
The following table may help to explain the situation:
If you are: then you are:
A born in the island of Ireland entitled to Irish citizenship or an Irish citizen.
B a child of A, born outside the island of Ireland an Irish citizen.
C a child of B and a grandchild of A, born outside the island of Ireland entitled to Irish citizenship, but you must first register in Foreign Births Register.
D a child of C and a great-grandchild of A, born outside the island of Ireland entitled to Irish citizenship, by having your birth registered in the Foreign Births Register, but only if your parent C had registered by the time of your birth.
When seeking to register in the Foreign Births Register, you will need to produce relevant documentation (birth and marriage certificates and other relevant records for yourself and those through whom you trace your Irish ancestry) to confirm your citizenship.