The Programme for International Student Assessment (PISA) assesses the performance of 15-year-old students in reading, mathematics, and science. In Ireland, information from PISA is highly valued by the Department of Education (DoE) as a guide to inform policy-making. Since Ireland first participated (2000), PISA testing has taken place in the Spring (March/April). This is a particularly busy time in schools for the 60% of PISA-eligible students who are in Third Year of post-primary education. As a consequence, moving PISA testing to the Autumn (October/November) has been on the agenda of Irish policy makers for some time. While research in the United States found no statistically significant difference in performance on PISA tests administered during the Spring and Autumn 2003, research exploring the issue in an Irish context had never been conducted. The research at the heart of this thesis was designed primarily to compare the performance of 15-year-olds across Spring and Autumn administrations of the PISA tests in Ireland. Data analysed came from administrations of PISA tests and background questionnaires in the Spring and Autumn of 2018, from bespoke questionnaires administered to subject teachers and from interviews conducted with school principals. Results indicated that, overall, there was no statistically significant difference between the performance of 15-year-olds in PISA tests administered across the two time periods. However, multi-level modelling involving the interaction between time of testing and gender indicated that testing in the Autumn was associated with lower mathematics scores for males. Of particular note was the statistically significant higher percentage of low-achieving males in the Autumn administration of the tests (21.4%) compared to the Spring administration (15.7%). Other outcomes with respect to similarities and differences in student engagement, motivation and wellbeing when measured in the Spring and Autumn are also presented in the thesis. The implications of the findings for policy and practice related to PISA testing as well as avenues for further research are discussed.