Taoiseach Bertie Ahern joined other EU leaders in Lisbon today in signing the controversial EU Reform Treaty.
The treaty effectively replaces the planned EU constitution brokered by Mr Ahern during Ireland's EU presidency in 2004.
The rights charter will be attached to the Lisbon Treaty and, if ratified, will be legally binding for all EU members except Britain and Poland, both of which obtained opt-outs.
So far, Ireland is the only country that will hold a referendum on the treaty. The other member states will ratify it through their parliaments. The referendum is due to be held in Ireland in the first half of next year.
Polls suggest many Irish voters are undecided or indifferent, and parliamentary ratification in Eurosceptical countries such as Britain cannot be taken for granted.
Today's signature ceremony took place at the historic Jeronimos Monastery in the Portuguese capital. It was attended by the 26 EU state leaders as well as European Commission president Jose Manuel Barroso and European Council president José Socrates. Britain's Prime Minister Gordon Brown, who was unable to attend, was represented by Foreign Secretary David Miliband.
Mr Ahern was accompanied at the ceremony by Minister for Foreign Affairs Dermot Ahern and Minister of State for European Affairs Dick Roche.
Many of the politicians, including the Taoiseach, will later travel on to Brussels for the European Council meeting tomorrow.
In a statement this morning, the Taoiseach said it was a "historic day" for the European Union.
"The Reform Treaty which we are signing today will provide the Union with the means to meet the challenges facing it as a Union of 27 Member States. It will allow Europe to do more for our people in the years ahead," he said.
"Ireland has been a huge beneficiary of EU integration and the union is crucial to our future well-being. That is why a more effective EU is in Ireland's best interests. It is why the ratification of the Reform Treaty will be a priority for Ireland."
Mr Ahern said a "stronger, more purposeful and more effective union" serves Ireland's interests.
"That is why I will be urging support for the Reform Treaty when it is put to the electorate for decision during 2008."
Yesterday, a group of jeering MEPs disrupted a ceremony in Brussels marking the signing of the EU charter of fundamental rights yesterday, using the opportunity to call for more referendums on the new EU reform treaty.
The MEPs heckled Portuguese prime minister José Socrates as he delivered a speech before signing the charter on behalf of EU member states. Independent Irish MEP Kathy Sinnott was among the protesters but said she did not heckle.