Adolescent self-harm is recognized as a serious public health problem; however, there is little reliable comparative data on its prevalence or characteristics, or on the extent of help-seeking for self-harm. The aims of the present study were to determine the prevalence and associated factors of adolescent self-harm in an urban region in Ireland, and to investigate help-seeking behaviours for self-harm. This was a cross-sectional study of 856 school-going adolescents, employing an anonymous self-report questionnaire. A lifetime history of self-harm was reported by 12.1% of adolescents. Factors independently associated with self-harm included exposure to self-harm of a friend/family member. Professional help-seeking was uncommon prior to (9%) and after (12%) self-harm. Furthermore, only 6.9% of adolescents presented to hospital as a result of their last self-harm act. These findings indicate that self-harm is common in adolescents; however, seeking professional help is not a common phenomenon, and those who present to hospital represent the ‘tip of the iceberg’ of adolescent self-harm. Identifying the prevalence of self-harm and associated factors, in addition to help-seeking behaviours, in young people is important to determine the preventative programmes to target ‘at-risk’ groups. Mental health nurses have an important and increasing role to play in such school-based initiatives.