Ratification process

Under the current rules for the amendment of the Treaties, the Treaties can only be amended by a new Treaty, which must be ratified by each of the Member States to enter into force.

In all the EU member states the Treaty of Nice was ratified by parliamentary procedure, except in Ireland, where the Irish Supreme Court in an earlier judgement on the Single European Act had ruled that fundamental changes to European Treaties, which alter the Irish Constitution's recognition of sovereignty as being ultimately derived from the People, require an amendment to the Irish constitution. Ireland's constitution can only be amended by a referendum of the people.

To the surprise of Europe's political classes, the voters in Ireland rejected the Treaty of Nice in June 2001. The turnout itself was low (34%), partly a result of the failure of the major Irish political parties to mount a strong campaign on the issue, presuming that the Irish electorate would pass the Treaty as all previous such Treaties had been passed by big majorities. However many Irish voters were critical of the Treaty contents, believing that it marginalised smaller states. Others questioned the impact of the Treaty on Irish neutrality. Other sections viewed the leadership of the Union as out of touch and arrogant, with the Treaty offering a perceived chance to 'shock' the European leadership into a greater willingness to listen to its critics. (A similar argument was made when Denmark initially voted down the Treaty of Maastricht.) In large measure, the Treaty of Nice was lost because pro-treaty supporters simply never bothered to vote, while the 'Vote No' campaigns were effective in raising serious questions as to the value of the Treaty.

The Irish government, having obtained the Seville Declaration on Ireland's policy of military neutrality from the European Council, decided to have another referendum on the Treaty of Nice on Saturday, 19 October 2002. Two significant qualifications were included in the second proposed amendment, one requiring the consent of the Dáil for "enhanced cooperation" under the treaty, and another preventing Ireland from joining any EU common defence policy. A 'Yes' vote was urged by a massive campaign by the main parties and – for the first time in European referendums in Ireland – by civil society and the social partners, including campaigning through canvassing and all forms of media by respected pro-European figures like then EP president Pat Cox, former Czech president Václav Havel, former President of Ireland Patrick Hillery and former Taoiseach (prime minister) Dr. Garret Fitzgerald. Prominent civil society campaigns on the Yes side included Fianna Fáil, Fine Gael, the Labour Party, the Progressive Democrats, the Irish Alliance for Europe led by Professor Brigid Laffan and Adrian Langan, and Ireland for Europe led by Ciarán Toland. On the No Side, the principal campaigns were those of the Green Party, Sinn Féin, Anthony Coughlan's National Platform, Justin Barrett's No to Nice campaign, and Roger Cole's Peace and Neutrality Alliance. The result was a 60% "Yes" vote on almost 150% of the turn-out of the previous referendum.

By then all other EU member states had ratified the Treaty. Ratification by all parties was required by the end of the year, or else the Treaty would have expired.