This article analyzes the significance of the 12 June 2008 Lisbon Treaty referendum in the Republic of Ireland. This was the third such referendum on Europe held in Ireland since the millennium, and the second referendum in three to result in a rejection of an EU Treaty following the failed Nice poll in 2001. Assessing both the campaign itself and the reasons for the No vote, the article argues that whilst variables such as age, educational attainment, geography, gender and social class all have a part to play in explaining the outcome of the referendum, post-referendum analysis suggests that two key phenomena proved decisive. First, an enduring Irish attachment to an overwhelmingly exclusivist national identity rather than more open and fluid identity conceptions, means that a space exists where issues such as neutrality, sovereignty and Ireland’s relative influence in the EU institutional matrix can be readily exploited by opponents of the European integration process, and where any changes in the EU constitutional order can be emotively presented as an existential threat to Ireland’s values and interests. Second, post-referendum analysis also suggests that lack of knowledge constituted a key reason for voting No. The absence of any effort by government to provide and promote sufficient information channels which explain how and why Ireland’s EU membership matters means that EU ‘debates’ within Irish political culture are frequently characterised by apathy, confusion, and ignorance, in a context where the chasm in elite-popular opinion has grown wider. The referendum result also points to a growing Eurosceptic tendency in Ireland which has seen the size of the No vote increase from 17 per cent in 1972 to a decisive majority of 53.4 per cent in 2008, on a significantly higher turnout than either 2001 or 2002.