The Irish referendum on the European Union's Lisbon Treaty, held on 12 June 2008, produced a negative outcome with the measure being rejected by the electorate by a margin of almost 54 per cent to 46 per cent. Thus for the second time in seven years Irish citizens declined to support a Treaty which their political elites overwhelmingly endorsed. The result plunged the European Union into yet another crisis, the latest following the rejection of the earlier Constitutional Treaty in referendums in France and the Netherlands in 2005. This article analyses the Irish referendum, the main actors and issues which influenced the campaign, and the reasons for the No vote. It argues that the two most significant phenomena which explain the result are the attachment of the Irish people to a particularistic conception of Irish identity combined with a lack of knowledge about both the EU decision-making system and the content of the Lisbon Treaty itself.