Strong evidence exists of a downward secular trend in the age at menarche; which appears to be associated with improved health and nutritional circumstances over the past two centuries. The aim of this study was to determine the mean age at menarche of Irish girls in 2006, as this has not been verified since 1986. A nationally representative sample of Irish girls (n = 4720, age 10-18 years) was obtained from the 2006 Irish Health Behaviour in School Aged Children survey and mean menarcheal age was calculated using the recall method. Mean age at menarche decreased from 13.52 years in 1986 to 12.53 years in 2006 and was lowest in cities and among manual groups. No significant differences were found between Irish-born and immigrant girls. Ongoing observation of this biological milestone is warranted to identify whether this secular trend will stabilise in Ireland and at what age. Consequences of earlier biological maturation on adolescent health behaviours and outcomes should also be monitored.