Objective: Tackling adolescent sexual health problems and promoting positive sexual health has been widely acknowledged in health policies across Ireland. Increasingly emphasis is placed on integrating empirical evidence at policy and practice level, however limited data on Irish adolescents’ sexual behaviour are available. This study presents the first nationally representative and internationally comparable data on adolescent sexual behaviour in Ireland. Design and Method: Questionnaire data were collected from a stratified national sample of 4494 schoolchildren aged 15-18 as part of the Health Behaviour in School aged Children study. Results: Overall, 28.8% of boys and 22.8% of girls were sexually initiated, with clear age and social class gradients. Substance use and living in poorer neighborhoods were predictive of sexual initiation. While there were no systematic urban/rural differences, living with both parents was protective. Very early sexual initiation (<14 years) was associated with male gender and rural dwelling, and for girls with smoking and cannabis use. Non-use of contraception at last intercourse was reported by 10.5% of boys and 6.8% of girls; with age and social class gradients. Condom use was predicted by self-care behaviours such as physical activity and seat-belt use, but not by substance use. Conclusions: For the first time, data have been collected which improve our understanding of adolescent sexual behaviour in Ireland. The findings highlight the importance of focusing on adolescents as a distinct population subgroup with unique influences on their sexual health requiring specifically targeted policy and interventions.