Objectives: Early childhood development strongly influences lifelong health. The Early Development Instrument (EDI) is a well-validated population-level measure of five developmental domains (physical health and well-being, social competence, emotional maturity, language and cognitive skills, and communication skills and general knowledge) at school entry age. The aim of this study was to explore the potential of EDI as an indicator of early development in Ireland. Design: A cross-sectional design was used. Setting: The study was conducted in 42 of 47 primary schools in a major Irish urban centre. Participants: EDI (teacher completed) scores were calculated for 1243 children in their first year of fulltime education. Contextual data from a subset of 865 children were collected using a parental questionnaire. Primary and secondary outcome measures: Children scoring in the lowest 10% of the population in one or more domains were deemed 'developmentally vulnerable'. Scores were correlated with contextual data from the parental questionnaire. Results: In the sample population, 29% of children were not developmentally ready to engage in school. Factors associated with increased risk of vulnerability were being male OR 2.1 (CI 1.6 to 2.7); under 5 years OR 1.5 (CI 1.1 to 2.1) and having English as a second language OR 3.7 (CI 2.6 to 5.2). Adjusted for these demographics, low birth weight, poor parent/child interaction and mother's lower level of education showed the most significant ORs for developmental vulnerability. Calculating population attributable fractions, the greatest population-level risk factors were being male (35%), mother's education (27%) and having English as a second language (12%). Conclusions: The EDI and linked parental questionnaires are promising indicators of the extent, distribution and determinants of developmental vulnerability among children in their first year of primary school in Ireland.