The incoming government must take action to stop children convicted of crimes being sent to St Patrick's Institution, the Ombudsman for Children 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.
"It's a very frightening thing to be in a prison in the first place, but it's also quite frightening to be housed where there are adult prisoners and we'd like it ceased," 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 today, 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.
Ms Logan said conditions in the cells should be improved, adding that young people's privacy should be respected and they should be able to shower more regularly.
"It is not enough to say that things will change for these young people at some future date," continued Ms Logan.
"The young people in St Patrick's Institution have offended and are paying the price for this through the deprivation of their liberty
"It is important that the state does all it can to promote and support the dignity of these young people while they are in the care of the state, and supports their reintegration into society.
"It is also important to note that the young people detained in St Patrick's Institution have been known to the state for some time, and that most have come into contact with a range of state services on many occasions before they reached St Patrick's."
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.