Mental Health Reform is calling for the urgent attention of government
Almost one in ten children admitted to adult mental health units in 2015 were aged 16 years or younger.
The Mental Health Reform group say they are "concerned" by the new data.
The figures are contained in the latest Health Service Executive (HSE) Performance Report, which says 95 children were initially admitted to an adult unit in 2015 - and of these nine or 9.5% were aged 16 or younger.
The December 2015 report revealed that overall 356 children required inpatient care in 2015 - and 261 of these were admitted to a child and adolescent mental health unit.
It says 95 of these were admitted to an adult unit.
The proportion of children admitted to adult units has decreased by approximately 5% since the end of 2013.
The figures also reveal that there has been an increase in the number of children and young people receiving acute inpatient mental health care, with the numbers increasing from 290 in 2014 to 356 last year.
The director of Mental Health Reform, Dr Shari McDaid, said: "These new figures serve to underline why mental health services need the urgent attention of government".
"At a time when all parties are negotiating the formation of the next government, Mental Health Reform would urge all those involved to ensure the provision of mental health services is prioritised in the new Programme for Government".
Dr McDaid also expressed concern that: "the lack of capacity in community based mental health services could be leading to an increase in children being admitted to inpatient care".