Under the Domicile and Recognition of Foreign Divorce Act 1986, it depends on whether or not the two parties to the divorce were domiciled in the jurisdiction of the Court granting the divorce.
If either of the parties to the divorce were domiciled in the jurisdiction of the Court, then the divorce is valid under Irish Law.
If neither party were domiciled within the jurisdiciton of the Court, or if they both had no fixed intention as to residence, then the divorce would not be recognised under Irish Law.
EDIT: Domicile is a complex legal concept regarding one's place of residence, or if emigrated, one's intended future residence. For example, if I was born in the USA, my domicile would default to the USA. If I stayed in the USA all my life, I would be domiciled there all my life.
However, if I emigrated to the UK, but it was my
intention to return to the USA someday, then I would be domiciled in the USA (because it would be my intended future place of residence).
If, while I was resident in the UK, I
intended to reside in the UK for the rest of my life, my domicile would change to the UK.
If, while I was resident in the UK, I had no fixed intention as regards residence, my domicile would automatically default back to USA.