A total of 165 children and teenagers were placed in adult prisons last year, in contravention of international treaties which prohibit the detention of juveniles alongside adults.
The Prison Service has confirmed to The Irish Times that 165 young people aged between 15 and 17 were placed in adult places of detention.
The majority were in St Patrick's Institution (which accommodates young people up to the age of 21), although others were placed in Limerick and Cork prisons during 2006.
One 13-year-old also spent a night in St Patrick's Institution in January of this year, before being removed home once authorities verified his age.
The Government has been rebuked by the United Nations Committee on the Rights of the Child on a number of occasions for allowing young people to be detained in the same units as adult offenders.
However, the Irish Prison Service says since new laws came into effect in March of this year, the practice of placing teenagers alongside adult offenders has begun to draw to a close.
It says all under-18-year-olds are now sent to St Patrick's, with the majority placed in new special school aimed at 16- and 17-year-olds.
This 44-bed unit, known as B division, was opened in April this year and has dining, training and recreational areas that are separate from those for young adults.
"This has created a separate and enhanced regime for them," a spokeswoman for the Irish Prison Service said.
However, when the number of teenage prisoners exceeds capacity, teenagers are accommodated in the drug-free D division of the main institution alongside young adult prisoners.
Also, a small number of 17-year-olds with specific individual needs are kept in the young adult part of the institution. These include sex offenders and vulnerable inmates who are accommodated separately for their own protection.
St Patrick's came in for strong criticism from the late Inspector of Prisons, Mr Justice Dermot Kinlen, who last year described the regime as a "disaster".
While workshops and training facilities have reopened, there appears to be a major ongoing problem with bullying. One fifth of the inmates have been placed on protection, which means they are kept in isolation cells for most of the day.
The latest figure available for this year show there were 57 teenagers in St Patrick's. The majority had committed offences relating to stealing cars, criminal damage, burglary and theft.
Most teenager offenders were from Dublin (17), followed by Limerick (14), Cork (4) and Louth (4).
When the figures for Dublin are broken down by postal district, they indicate teenagers were from more deprived parts of the capital such as the south inner city (four offenders were from Dublin 8), followed by the Tallaght area (three offenders were from Dublin 24).
The Government has approved reforms that will see the closure of St Patrick's and the increased use of correctional schools for under-18s by 2010. A number of such facilities already exist, such as Trinity House in north Dublin.
These plans have been welcomed by groups such as the Children's Rights Alliance - a coalition of 80 children's rights groups - although it has voiced concern at the Government's timetable, which may not result in the closure of the institution for at least another three years.
The number of young people in adult places of detention is likely to reduce dramatically this year following the introduction of a provision in the Children Act (2001), under which St Patrick's was designated as the sole institution to which 16- and 17-year-old males may be committed on remand or sentence.