Background: We examined the role of neighbourhood characteristics in explaining socioeconomic inequalities in child mental health in Ireland.Methods: Data from Cohort ’08 of Growing Up in Ireland with Wave 3 (age 5) as the baseline and Wave 5 (age 9) as the follow-up (n=8,373 for cross-sectional and n=6,349 for longitudinal analyses) were used. Socioeconomic status (SES) indicators were caregiver education, occupation, and equivalised household income. Caregiver and teacher reports of total difficulties score (TDS) from the Strength and Difficulties Questionnaire were combined with a higher score indicating more mental health difficulties. Twenty neighbourhood items rated by caregivers were grouped into five domains (neighbourhood safety, built environments, cohesion, interaction, and disorder) using exploratory factor analysis. We used linear regression to assess the associations between SES indicators, neighbourhood characteristics, and TDS, controlling for sociodemographic covariates. Mediation analysis informed the extent to which (%) neighbourhood domains explained the SES-TDS associations. Network psychometric analysis was used to identify neighbourhood items that were most strongly linked to TDS.Results: Lower SES was consistently associated with a higher baseline, follow-up TDS, and longitudinal changes in TDS. Favourable neighbourhood domains (higher safety, cohesion, interaction, lower disorder) were associated with lower baseline and follow-up TDS. Neighbourhood safety, cohesion, and disorder were also associated with longitudinal changes in TDS across waves. Four neighbourhood domains (except built environments) explained the associations between SES and baseline and follow-up TDS by 3% to 18% in separate single mediation models, and these four domains in concert explained 12% to 23% in multiple mediation models. Built environments domain predicted TDS and mediated the associations between SES and baseline TDS (5% to 11%) in urban children only. No mediation of the associations between SES and longitudinal changes in TDS by neighbourhood domains was observed. Findings from network analysis indicated that specific concerns over “people being drunk or taking drugs in public” and whether “this is a safe neighbourhood” may act as key conceptual bridges between neighbourhood characteristics and TDS.Conclusions: Neighbourhood characteristics may explain cross-sectional socioeconomic inequalities in child mental health. Improving neighbourhood characteristics where lower-SES children live may be important to reduce SES-based inequalities in child mental health.