This Growing Up in Ireland report is a supplemental analysis to the previous publication Key Findings: Cohort ’08 at 13 years old (2023). This supplement discusses the findings emerging from the self-complete online module from that survey (Cohort ’08 at 13), whereas the initial report was concerned with an analysis of information from the main telephone interviews with participants at that time. The self-complete surveys, and therefore this report, cover more sensitive topics such as relationships, mental health and substance use; which participants may not have felt comfortable discussing over the telephone. There were separate modules for 13-year-olds and their parents.