Almost 300 children and adolescents were admitted or detained in adult psychiatric hospitals last year because no other suitable facilities were available.
Children as young as nine have been placed in adult facilities, despite warnings from patients' groups and psychiatrists that the practice is adversely affecting children.
The official figures, seen by The Irish Times and due to be published in November, come on the eve of the full implementation of the Mental Health Act (2001), which aims to improve services for young people with psychiatric problems.
However, health authorities acknowledge that the practice of admitting young people to adult units is likely to continue because there is a shortage of child and adolescent units.
There are between 20 and 25 inpatient beds for young people with mental health problems, yet successive reports to Government have recommended that at least 120 beds should be provided.
A Health Service Executive (HSE) spokeswoman said that with the full implementation of the Act, each region was being asked to identify three to four suitable beds on an interim basis.
Meanwhile, staff in adult facilities will be provided with training to help meet the needs of children.
Dr Mandy Burke of the Irish College of Psychiatrists said the issue had reached crisis levels.
"It's not a safe practice, but it's the best we have to offer. In this day and age we should have age-appropriate units. Adults' facilities should only be used in case of dire emergency."
Minister of State for Health Tim O'Malley said he was hopeful interim measures would address most of the demand for age-appropriate services.
He said plans for four new child and adolescent units in Cork, Limerick and Dublin were at an advanced stage and should be completed within 2½ years.
Official figures will show the number of young people admitted to adult units has varied between almost 400 in 2002 to under 300 in 2004 and 2005.
Most admissions last year were voluntary (90 per cent), although 10 per cent were against the will of the child's parents.
