Background: The aim of this study was to use a social-ecological approach to examine the influence of individual, social, and environmental factors on moderate-to-vigorous physical activity (MVPA) and screen-time in a sample of 9-year-old children in Ireland. Methods: The sample was 1509 boys and girls from the Growing Up in Ireland (GUI) national study. MVPA, screen time, and individual, social, and environmental variables were assessed via questionnaires completed by children, their parents, and their teachers. Multiple regression was used to identify factors that correlated with children’s MVPA and screen-time levels. Results: For boys, factors such as activity with friends (P < .0001) and popularity (P < .01) were associated with MVPA, while factors such as BMI (P < .01) and MVPA (P < .01) were associated with screen time. Similarly for girls, factors such as activity with friends (P < .0001) and sociability were associated with MVPA, however factors such as BMI (P < .05), and access to play space (P < .05) were more closely associated with screen time. Conclusion: Social factors were more closely associated with MVPA, while individual factors were significantly correlated with screen time for both boys and girls. Correlates differed for boys and girls, suggesting that interventions should consider both the target population as well as the activity behavior.