One in four children are unfit, overweight or obese and have high blood pressure, research has revealed.
A study for the Irish Sports Council (ISC) has uncovered alarming levels of inactivity among 10 to 18-year-olds inside and outside school.
While three-quarters play sport twice a week after classes, only a fifth of primary children and 12% of post-primary get the recommended one hour of intense workout a week.
And less well-off youngsters are much less likely to take part in activities or extra-school sport than middle-class children.
Mary Hanafin, Sports Minister, said the study showed some successes but also highlighted areas where work needed to be done.
"The research also shows there remains a clear challenge to keep young people, especially girls, engaged in active sports through their teens," she said. "The key message is to use every opportunity to get and keep children and young people active through a combination of participation in school and extra school activities, through involvement in local clubs and in quality active leisure time with their family and friends."
The Children's Sport Participation and Physical Activity Study (CSPPAS) found gender gaps remained in fitness levels, with girls less likely than boys to meet recommended physical activity guidelines while participation rates also fall as children get older.
Researchers have found the number of children walking and cycling to school is increasing but said it remained at worrying low levels - 38% last year compared to 42% in 1991.
John Treacy, ISC chief executive, added: "It reinforces many of the key outcomes of the 2004 research and again highlights contribution of sport to the health and wellbeing of our children."
The researchers have set a series of targets for child fitness, including getting more than half walking or cycling to school and half of primary children doing one hour of intense exercise a week.