Background: Higher levels of sedentary behaviour (SB) and screen-time are associated with greater symptoms of depression in adolescents, but the effect of the type and context of SB and screen-time remains underexplored. As part of a nationally-representative observational study, the current cross-sectional study examined associations between SB, screen-time and depressive symptoms among 422 adolescents (13.5 ± 0.92 years; 125 female) in the Republic of Ireland. Method: Participants completed the Quick Inventory of Depressive Symptomatology and self-reported weekly SB, categorised into mentally-active screen-time (e.g., computer use for fun), mentally-passive screen-time (e.g., television viewing) and mentally-active non-screen-based SB (e.g., reading). Mann¿Whitney U tests and Kruskal¿Wallis H tests examined differences in screen-time and depressive symptoms by relevant covariates. Linear regression quantified crude and adjusted associations between total SB and mentally-active and mentally-passive screen-time and SB, and depressive symptoms. Results: Crude and adjusted linear regressions showed total SB was significantly, positively associated with depressive symptoms (unadjusted: ß = 0.27, p = 0.002, adjusted: ß = 0.27, p = 0.002). When type and context were examined in the same model, only mentally-active screen-time was positively associated with depressive symptoms (unadjusted: ß = 0.37, p = 0.009, adjusted: ß = 0.39, p = 0.007). Conclusion: Differential associations between total SB and mentally-active screen-time and SB, versus mentally-passive screen-time, and depressive symptoms among Irish adolescents were observed. Findings highlight the importance of investigating the context and type of SB and screen-time in adolescents.