Inmates in juvenile detention centres cost the Irish State almost 10 times the European average in 2014, according to the Council of Europe.
Figures from the council’s annual penal statistics show the cost per inmate per day in an institution for juvenile offenders was €2,773.38.
This was by far the highest figure across Europe and compares to €897.65 in Northern Ireland, €303.48 in the UK and a Europe-wide average of €283.58.
Pat Bergin, director of Oberstown Children Detention Campus, said the cost per placement per year at the facility in 2015 was €340,983. This equates to €934 per day.
The council of Europe report also said 10.9 per cent of final sentenced inmates in Ireland were serving life imprisonment compared to Europe-wide average and median figures of 3.5 per cent and 1.8 per cent respectively.
The average age of prisoners in Ireland on September 1st, 2015, was 34. Only France had a lower average age at 33.8.
Among western European countries, Ireland and the UK were among those with the lowest proportion of foreign inmates.
A relatively high proportion of prisoners serving final sentences in Ireland had been convicted mainly for assault and battery (13.8 per cent), rape (5.1 per cent) and other sexual offences (6.2 per cent).
This compared to Europe-wide averages of 9.3 per cent, 3.9 per cent and 3.7 per cent respectively.
In terms of the overall findings, the number of people held in penitentiary institutions across Europe fell by 6.8 per cent to 1,404,398 between 2014 and 2015.
Nevertheless, overcrowding was found to be a problem in 15 of the 45 institutions surveyed, notably including EU member states Spain, Hungary, Belgium, France, Portugal, Italy, Austria and Romania.
Across Europe, the median cost per inmate per day in 2014 was €52, with values ranging from €6 per prisoner per day in Georgia to €481 in San Marino.
The total amount spent by the 44 prison administrations that provided data was more than €21 billion.
In 2015, inmates serving final sentences represented 73 per cent of the total prison population. Among those prisoners, almost one in five was convicted for drug-related offences.
This proportion was relatively high in EU members Italy (31.1 per cent), Estonia (28.3 per cent), Cyprus (27.5 per cent) and Luxembourg (26.2 per cent).
In 2015, foreign inmates represented 10.8 per cent of the total prison population, down from 13.7 per cent in 2014. This proportion continued to be very small in central and eastern European countries, whilst it was relatively large in western Europe.