President Mary McAleese has signed the Bill of the Lisbon Treaty giving legal effect to the referendum vote.
Mrs McAleese signed the document at Aras an Uachtarain following her return from a State visit to Luxembourg.
A statement the Aras said: "President McAleese has signed the Twenty-Eighth Amendment of the Constitution (Treaty of Lisbon) Bill 2009 in accordance with Article 46 of the Constitution."
The instrument will now be deposited in Rome which will complete the ratification process in advance of an EU summit in Brussels at the end of the month.
This follows 67 per cent Yes vote in the referendum two weeks ago.
