The meteoric rise and catastrophic fall of Ireland’s economy between 1997 and 2010 is a compelling story. The so-called Celtic Tiger disappeared and Ireland turned to international partners for financial assistance. This research seeks to understand the causes of the economic crash in Ireland, and - more specifically - to better understand one of the regularly cited causes: poor economic decision-making. The puzzle for this research is to reveal which factors influenced policy-makers in Ireland and how those factors impacted decision-making. This includes better understanding the presence and impact of irrationality in policy-making. The research seeks to determine (i) whether evidence of active, impactful irrationality among key decision-makers in the period before Ireland’s crash is detectable using a quantitative method, and linked to this (ii) what role did interests, institutions, and ideology play in poor decision-making. The research proposes that leaders in Ireland fell victim to higher relative levels of a particular form of decision-making bias, behavioural convergence, and that this can be tested using the leadership traits conceptual complexity and in-group bias as a proxy, employing the Leadership Trait Analysis technique. Contrary to the hypothesis, the research finds that Irish leaders in the run up to the crisis had higher rather than lower scores for conceptual complexity, and lower rather than higher in-group-bias, relative to other leaders. The thesis finds that personality trait scores vary considerably over time, that the position that the leader holds, and that the decision-making context can have a significant impact on leaders’ trait scores. Further, the nature of the content analysed (ad libbed versus pre-prepared text) can also have a significant impact on scores. Next, process tracing a decision case-study finds evidence of interests, institutions, and ideology impacting on decision-outcomes. However, it is the combination of ideological and institutional factors that have the clearest negative impact. The research brings these learnings together to help inform better decision-making, and indicates scope for further research arising from this thesis.