Taoiseach Bertie Ahern will join other EU leaders in Lisbon tomorrow to sign the controversial EU Reform Treaty.
A referendum on the treaty, which effectively replaces the planned EU constitution brokered by Mr Ahern during Ireland's EU presidency in 2004, is due in the first half of next year.
France and the Netherlands subsequently rejected the constitution proposal in referendums.
Ireland is the only member state which must go to the people before tomorrow's signing by the Taoiseach is ratified. Ireland is therefore likely to be the only state to opt for a referendum on what will be known as the Treaty of Lisbon.
Former minister for finance Charlie McCreevy last week said failure to ratify the treaty would make Ireland "the laughing stock of Europe".
Mr McCreevy, who is now Internal Markets Commissioner, said the reform of EU institutions was essential for the effective administration of the Union.
An Irish Times/TNS mrbi poll last October found just 25 per cent of voters in favour of the treaty. But only 12 per cent were opposed with the remainder undecided.
Government ministers acknowledged the poll findings were important but insisted that that once an information campaign is launched, enough of the public will be persuaded to vote yes.
A further impediment to Ireland's ratification could be the Green Party, who have traditionally opposed expanding the EU's powers. The junior coalition partner will put the matter to a vote of their membership in January but the leadership is looking for members to support the treaty.
But there will still be party political opposition from Sinn Féin. Its only MEP in the Republic Mary Lou McDonald said today that Ireland has a place in the EU but that the treaty contained a "democratic deficit" and accelerated the "militarisation of the EU".
"The Lisbon Treaty will involve the most substantial transfer of powers from member states to the European Council and Commission to date. The influence of smaller states will be reduced as the dominance of the larger states is consolidated," she said.
Tomorrow's signature ceremony takes place at the historic Jeronimos Monastery in the Portuguese capital. It will be attended by the 27 EU state leaders as well as European Commission president Jose Manuel Barroso, European Parliament president Hans-Gert Pottering and European Council president Jose Socrates.
Afterwards the EU leaders are expected to pose for the traditional family photograph. Many of the politicians, including the Taoiseach, will later travel on to Brussels for the European Council meeting on Friday.
Meanwhile the 50th anniversary of the Treaty of Rome was marked in Dublin today. Minister for Foreign Affairs Dermot Ahern launched the publication 'Foundations of an Ever Closer Union' for the occasion.
The Treaty of Rome, which established the European Economic Community (EEC), came into force on January 1st, 1958.
Additional reporting PA