© 2013 © The Author 2013. Published by Oxford University Press on behalf of the Hansard Society; all rights reserved. The growth in the use of referendums to make major political decisions internationally has brought renewed interest in the factors which underpin voting behaviour at these types of elections. Referendums vary from traditional political contests, in that they are usually focused on a single issue, the dynamics of political party interaction can diverge from national and local elections, non-political actors may often have a prominent role in the campaign and voters may or may not have strong, clear views on the issue being decided. The LeDuc (2002, European Journal of Political Research, 41, 711-732) framework classifies referendums along a spectrum from stable to volatile. This case study is focused on the volatile end of the spectrum and will consider the Parliamentary (Oireachtas) Inquiries (OI) referendum in the Republic of Ireland. The OI referendum was defeated by a narrow margin and the campaign period witnessed a sharp fall in support for the proposal. This work employs two models of voting behaviour to understand the OI campaign, campaign learning and rational voting. Consistent with expectations for volatile referendums, the analysis shows that voters relied significantly on heuristics or shortcuts emanating from the campaign and to a lesser extent on either media campaigns or rational knowledge.