BACKGROUND: Childhood obesity is a growing concern in Ireland. Childhood obesity can increase the risk of developing many non-communicable diseases and have lasting psychological and social consequences. AIM: The aim of this study is to explore if weight at birth and breastfeeding status influence the weight status of 3-year-olds in an Irish cohort. METHOD: This retrospective cohort study utilised data (National Child Health Screening Programme) on 3-year-olds born between 1 January 2011 and 3 March 2014 in the North West of Ireland. RESULTS: Overall, 4144 children were included in the study. The main findings of this study were that 5.4% (n = 222) and 7.1% (n = 296) of the cohort were overweight or obese respectively with a higher percent of males than females in each category. Just under 55% (n = 2266) of the cohort were never breastfed, with only 7.4% breastfed for greater than 6 months. Those born with a high birthweight were more likely to be overweight or obese at 3 years (p ≤ 0.001). CONCLUSION: These findings provide regionally specific data and highlight the need for focussed public health efforts to reduce the prevalence of overweight and obesity in children aged 3 years in this area. Interventions from pregnancy through childhood are warranted, with an initial emphasis on breast feeding initiation and maintenance.