The referendum, held on 12th June 2008, rejected the Lisbon Treaty by a margin of 53.4% to 46.6%.
A total of 752,451 people voted in favour of the Treaty and 862,415 voted against.
The 'Yes' side was backed by the main political parties in Ireland, as well as business, farmers and labour unions.
The 'No' side was defended by a very mixed bag of minority groups, whose reasons for opposing the Treaty were varied.
Sinn Féin was the only party represented in the Irish Parliament to campaign for a 'No' vote, on the grounds that the Treaty was a “bad deal for Ireland, for the EU, and for the developing world”, according to its president Gerry Adams.
Catholic groups called for a 'No' vote over fears that the Lisbon Treaty would overrule Irish laws making abortion legal in Ireland.
Coir, another campaigning group against the Treaty, argued the EU plan would seriously dent Ireland's national sovereignty.
The Peace and Neutrality Alliance campaigned for 'No' on concerns over a militarised Europe and the eventual loss of Ireland's traditional neutrality.
Pro-democracy group Libertas opposed the Lisbon Treaty on claims that the EU should be more transparent and accountable.
Independent politicials and pro-free market groups also campaigned for a 'No' vote on fears over a possible tax harmonisation which would harm the Celtic Tiger's advantage as a low tax economy.
The Lisbon Treaty was an attempt to continue with EU reforms after the project for an European Union Constitution was aborted in 2005.
Ireland was the only EU country to ask its citizens their opinion in a referendum. The rest of the EU countries are ratifying the Lisbon Treaty through their national parliaments.
EU shock as Ireland derails Lisbon Treaty
Politicians in Ireland and Europe were not expecting the Treaty to be rejected in referendum by one of the EU's most pro-European nations. Irish bookmakers were also overwhelmingly expecting the referendum to pass.
As EU leaders were left shocked with no clear answer over what to do next, the international press announced that the Irish 'No' vote opened a new crisis in the Union.
The referendum result was interpreted as an embarrassment for the Irish leaders, as well as for France and Germany, who masterminded the new Lisbon Treaty last year and were expecting to give a new direction to the EU during the French presidency.
EU leaders will now have to find a solution to the derailed Lisbon Treaty. This could involve another renegotiating round among all the EU member states or Ireland being exempt from the Treaty for now.
Majority parties refuse to speculate on second referendum
This was not the first time Irish voters have rejected an EU treaty in referendum. In 2001 Ireland rejected the Nice Treaty on EU Enlargement, although it was later passed on a second referendum.
But this time, Taoiseach Brian Cowen as well as the Fine Gael and Labour Party leaders refused speculate about holding a second referendum at a later stage, arguing the Irish people had made a clear-cut decision on the matter.
"In a democracy, the will of the people as expressed at the ballot box is sovereign. The government accepts and respects the verdict of the Irish people", declared the Taoiseach.
Libertas leader says result “is a pro-European message”
The 'No' vote was importantly boosted up by the campaign of the recently created lobby Libertas, which defines itself as a “new European movement dedicated to campaigning for greater democratic accountability and transparency in the institutions of the EU.”
Libertas' founder, businessman Declan Ganley, celebrated the referendum result and insisted that the Irish 'No' vote was not an Eurosceptic vote but a “pro-European message.”
Mr Ganley said there had not been enough information about the consequences of the Lisbon Treaty and highlighted the Treaty would have provided for an unelected President.
He claimed the EU should be more democratic and accountable and asked “Will Brussels listen to this message or are they going to seek to find a way around this result?"
"That's the message Irish people have given today and it makes me very proud to be Irish and very proud to be European and a pro European to see this message go out", told Declan Ganley.