The report also highlights the ongoing admission of children to adult psychiatric units contrary to the code of practice relating to the admission of children under mental health legislation.
Of 96 child admissions to adult units last year, 33 were aged under 16, six were aged under 15, and three were aged 13 or younger.
The Mental Health Commission, the national watchdog for psychiatric services, said in 2011 the practice should cease by the end of that year.
Director of Mental Health Reform, Dr Shari McDaid, said they were concerned about the ongoing admission of children to adult units which “shows the need for more staff to be recruited for child and adolescent community mental health teams”.
Three in five of the child admissions were female. Girls accounted for 87% of admissions with eating disorders.
Boys accounted for three in five with a primary admission diagnosis of schizophrenia, and 93% of admissions with other drug disorders.
Girls accounted for 65% of all admissions with a primary admission diagnosis of depressive disorders.
Wicklow recorded the highest admission rates in the country of 497.7 per 100,000, followed by Roscommon, Sligo, and Donegal, while Monaghan recorded the lowest rate of 211.6 per 100,000. Dublin ranked sixth highest, while Cork came in ninth.
Almost three in five admissions were for single people. While divorced people accounted for only 4% of all admissions, they had the highest rate of all admissions at 759.9 per 100,000.
Almost 41% of all admissions in 2015 were unemployed. Some 94% of all discharges occurred within three months of admission.
The figures, in the Activities of Irish Psychiatric Units and Hospitals 2015 report, show 18 to 19-year-olds had the highest first admission rate, in line with much of the last 10 years.
Young people between 20 and 24 years of age had the highest rate of all admissions at 577.7 per 100,000 population.
Depression continues to be the most common diagnosis for all admissions.
The report shows a small increase in admissions from 17,797 in 2014 to 17,860 in 2015. First admissions also increased by 172, from 5,942 in 2014 to 6,114 in 2015.
However, looking at trends over time, there has been a 12% decline in total admissions in the 10-year period from 2006 to 2015, from 20,288 in 2006 to 17,860 in 2015.
In line with the policy to close the older psychiatric hospitals, admissions to psychiatric hospitals/continuing care units continue to decrease.
In the 10-year period from 2006 to 2015, admissions to psychiatric hospitals dropped by 50% from 6,400 in 2006 to 3,187 in 2015.
Admissions to general hospital psychiatric units increased by 3%, and admissions to independent private and private charitable centres increased by 12% in the same period.
Dr Graham Love, chief executive at the HRB said the report “provides essential data to inform decision-making in relation to planning for mental health services”.
For more information, visit www.hrb.ie.