A large body of evidence suggests that adolescent mental health has worsened in recent years, particularly amongst young women. We investigated three putative risk factors which are very prevalent in the modern-day life of adolescents: social media use, sexting, and body dissatisfaction We wanted to investigate how these potential risk factors influence adolescents' mental health, and whether the effects differ between genders.We used a population-based survey of predominantly students (mean age 15 years) conducted in secondary schools in Ireland (September-December 2021) in one urban area (North County Dublin) and two rural areas (Cavan, Monaghan) - The "Planet Youth North Dublin, Cavan and Monaghan study". We examined three mental health outcomes (using the Strengths and Difficulties Questionnaire (SDQ)) and self-harm behaviours. We fitted linear mixed models to examine associations between risk factors and mental health outcomes in both genders separately, adjusting for confounders.All three risk factors were associated with poorer mental health in both genders, with larger effects in females. High social media usage (>4 hrs/day) was associated with increased poor mental health risk. Body dissatisfaction was linked to self-harm and worse mental health, being more prevalent in females (60% vs 36%). Sending sexually explicit messages was associated with poor mental health and self-harm risk in both genders.Our findings show that high digital use, body image disturbance and sexting behaviours in teenage life have a strong association with poor mental health and self-harm, with these factors being more prevalent in females.