A seven-year study of anorexia nervosa at the Children's University Hospital, Temple Street has found that the average age at admission was just under 14 years.
Among the 20 children, 70% were girls and the average hospital stay was 38 days.
50% were treated with antidepressants, according to the study in the latest Irish Medical Journal.
Anorexia nervosa is a serious mental illness and people with the condition in Ireland are presenting at an earlier age.
30% of the admissions were boys and there is an increased male prevalence.
The researchers say anorexia nervosa may look different nowadays and specialists must be ahead of the curve, given its high mortality rate.
Girls present to hospital sooner than boys, suggesting better awareness and detection of anorexia nervosa in girls.
The study says the new National Paediatric Hospital will need dedicated beds and specialist medical, nursing and dietetic posts for the illness.
The study examined the period 2005-2011 and says the large increase in admissions to the hospital over the past decade mirrors UK figures.
The patients in Ireland were admitted on a non-negotiable meal plan with total bed rest and usually supervision.
Aside from low weight, over-exercising and food restricting were the most common presenting features.
35% of the children had undergone recent orthodontic work before the onset of the illness.