OBJECTIVES This study provides data on the prevalence of mental health problems among adolescents in Ireland in 2021, toward the end of the COVID-19 pandemic. The importance of having recent, large-scale, mental health data for adolescents has been heightened by COVID-19, the increased demand for child and adolescent mental health services, and the rapidly changing adolescent environment.

METHODS As part of the Planet Youth study, a cross-sectional survey of adolescents ( = 4,404), mostly aged 15-16, was conducted between September and December 2021. Participants were recruited from 40 schools and non-traditional educational centres across 3 regions in Ireland, one predominantly urban (North Dublin) and two predominantly rural (Cavan, Monaghan). A range of mental health outcomes were self-reported: a single-item question on mental health; the Strengths & Difficulties Questionnaire (SDQ); depressive and anxiety symptoms from the Symptom Check List 90; the Adolescent Psychotic-like Symptom Screener; and lifetime self-harm, suicidal ideation, and attempt.

RESULTS Over a quarter of adolescents described their mental health as 'bad' or 'very bad' (29%), and had SDQ total problem scores over 20 (26%). Over a third (39%) reported self-harming, 42% reported suicidal ideation, and 11% reported attempting suicide, in their lifetime. Gender-diverse youth (non-binary, trans, and undisclosed) had higher rates of poor mental health outcomes compared to cis-gendered youth (male/female), and females had higher rates of most mental health outcomes compared to males.

CONCLUSIONS Many of these estimates suggest a deterioration from previous epidemiological studies. While our findings do not definitively prove youth mental health has worsened over time, these findings are highly concerning. We propose a close monitoring of mental health in future surveys of this population and encourage initiatives to improve the capacity and quality of youth mental health services.