This study investigated the relationship between school sex education and sexual health behaviours at first sex and later in adulthood, using nationally representative data. Respondents were adults from the 2010 Irish Contraception and Crisis Pregnancy Survey, a cross-sectional survey designed to assess knowledge, attitudes and behaviours relating to sex, contraception and pregnancy in Ireland. Results indicate that respondents who received sex education were more likely to have first sex at an older age and use contraception on this occasion. Sex education also significantly increased the likelihood of using contraception at first sex, when first sex occurred before 17 years of age. The effect of sex education and sexual health behaviours into adulthood was also investigated. Sex education increased the likelihood of having a history of sexually transmitted infection testing and decreased the likelihood of having experienced a crisis pregnancy. No association was found between sex education and contraception use over the past year. Findings suggest that sex education is an important factor in the context of first sex and later sexual health, and programmes should continue to equip young people as they make immediate sexual behaviour decisions and further sexual health-related decisions throughout their lifespan.