The incoming government must take action to stop children convicted of crimes being sent to St Patrick's Institution, the Ombudsman for Children has said.
Emily Logan demanded teenagers under the age of 18 be moved from the harsh environment of custody to one of care.
However, it will be at least another three years before phase one of a new national child detention facility is due to open at Oberstown near Lusk in Co Dublin.
Ms Logan said while most child prisoners are detained in one division, a small number put on protection are moved to another wing alongside adult inmates.
She said: "The dynamic in a prison is very different. It's a regime of custody. It's not the same as having a young person in care where they are able to get therapeutic interventions and there are options in terms of reintegration and rehabilitation in a care environment."
A total of 215 inmates between 16 to 21 years are in St Patrick's Institution, including 38 juveniles under the age of 18. Its capacity is 217.
Brian Purcell, director general of the Irish Prison Service, said the reality was that juveniles had to go to St Patrick's until the new facility is ready in Oberstown.
"It's very difficult to provide the type of services required for 16 and 17-year-olds in what is essentially a Victorian era prison and it is a secure detention facility," he said.
Ms Logan, who can not legally investigate cases from under-18s in St Patrick's, met 35 young inmates to review their care at the institution. She found they had to rely on fellow inmates to find out about prison life when they first arrived and were reluctant to speak up about mental health difficulties.
Mr Purcell maintained a new induction course has been established with only one protection prisoner under 18 living in an adult division for his own safety.