BACKGROUND: Research has demonstrated positive associations between adverse childhood experiences (ACEs) and alcohol use behaviours in young adults. However, many of these are based on cross-sectional data. The pathway between ACEs and alcohol misuse among emerging adults is not fully understood and there is a lack of data on how parental and peer relationships may mediate this relationship. OBJECTIVE: To examine ACE exposure relationships with alcohol use behaviours and determine if associations between ACEs and harmful alcohol use are mediated by parent and peer relationships and school engagement. PARTICIPANTS AND SETTING: Participants (n = 4729) were a cohort of children recruited to the Growing Up in Ireland study, a nationally representative longitudinal sample of children. METHODS: Linear and logistic regression analyses examined individual and cumulative ACE exposure relationships with alcohol use behaviours at age 20 years. Mediation analyses assessed whether parental and peer relationships and school engagement mediate ACE history associations with harmful alcohol use. RESULTS: Exposure to ACEs in childhood was associated with higher Alcohol Use Disorders Identification Test (AUDIT) scores and alcohol dependence at age 20. Young adults who experienced 3+ ACEs had a three-fold (OR = 3.33, 95 % CI: 2.00, 5.55) increased odds of alcohol dependence when compared to participants who experienced no ACEs. Mediation analyses suggested that parent and peer relationships partially mediate associations between ACEs and harmful alcohol use behaviours. CONCLUSIONS: Children who have experienced ACEs are at higher risk of harmful alcohol use. In supporting these children, identified mediators may help guide selection of prevention interventions.