Objectively measured and self-reported physical activity levels were assessed among Irish adolescents (n=28) by heart rate monitor and self-report questionnaire. The purpose was to determine the percentage of Irish adolescents that met international physical activity guidelines using both objective and subjective measurements. More than 11% of subjects met the international moderate intensity physical activity guidelines for adolescents when measured by self-report questionnaire. No subjects met this guideline when measured objectively by heart rate monitor. Levels of agreement between the two measurements were poor (k < 0.40). When measured objectively by HR monitor, no adolescent met the international guidelines on sustained vigorous physical activity. Prevalence estimates for compliance with international physical activity guidelines varied according to mode of measurement. We conclude that compared to self-report measures, adolescent physical activity levels are lower than those required for health benefits when measured objectively, and only a minority of young people satisfy the international recommendations.