Anxiety and depression are among the most common mental health concerns in youth, with rates of these internalizing problems continuing to rise. Universal school-based interventions have shown promising results in improving poor mental health outcomes; however, more research is needed across different cultural contexts. This study is part of an ongoing evaluation of "A Lust for Life Schools Programme," a universal process-based cognitive behavioral therapy intervention for Irish primary school children. This study investigated the efficacy of the program through a cluster randomized controlled trial. Nine schools were randomly allocated to the intervention or control groups. Four hundred and seventy participants completed measures at three timepoints (baseline, post-intervention, and 6-week follow-up) assessing internalizing problems, anxiety, and depressive symptoms (primary outcomes) and avoidance, problem-solving, seeking social support, academic self-efficacy, social self-efficacy, and emotional self-efficacy (secondary outcomes). Although participants reported that they were mostly satisfied with the intervention, results revealed that the intervention did not have a significant effect on the outcome variables compared to the control group. Implications of the findings and recommendations for future research are discussed.