The PISA 2009 results for Ireland indicated a large decline in reading literacy scores since PISA 2000 (the largest of 38 countries). The decline in mathematics scores since PISA 2003 was the second largest of 39 countries. In contrast, there was no change in science achievement since PISA 2006. These results prompted detailed investigations into possible reasons for the declines, particularly in reading. This paper considers the changes in achievement observed for Ireland in PISA 2009 under two themes: implementation of PISA in Ireland and changes in the cohort of students participating in PISA, and response patterns on the PISA test (as measures of student engagement). It is argued that the case of Ireland represents the 'perfect storm', since a range of factors appear to have been in operation to produce the results. The discussion attempts to show how the case of Ireland can be relevant to other countries which may have experienced changes in PISA test scores over time. Some of the findings have relevance to international practice in large-scale surveys of educational achievement more generally.