One in five adolescents is at risk of psychiatric problems, according to the first major study of suicidal behaviour and psychiatric disorders among young people.
The study, among pupils attending eight secondary schools in Dublin, found that girls attending co-ed schools were twice as likely to be at risk.
The report found that young people living with both their parents were significantly less likely to be at risk.
But girls living with their mother and a partner were five times more likely to have problems.
Boys living with extended family or in foster care were twice as likely to be at risk of psychiatric problems.
Details of the study were presented at a public lecture in Dublin last night by Prof Carol Fitzpatrick, professor of child psychiatry at University College Dublin.
"Its background lies in the high rate of youth suicide in Ireland, particularly among young men in the 15-to-24 age range," she said.
"The study set out to move behind the statistics, to look at younger adolescents to determine rates of depressive disorders and suicidal behaviours in this age group."
Prof Fitzpatrick said this type of information was essential to help plan and evaluate the effectiveness of programmes aimed at preventing suicide and suicidal behaviours.
The study sample was of 723 adolescents, 22 of whom were referred to child and adolescent mental health services as a result of the study's findings.
Questionnaires were used to establish the pupils' moods and whether they could be considered at risk of a psychiatric disorder. Some 140 students or 19 per cent of the cohort were found to be in this category.
This group was therefore questioned further. These pupils were interviewed for an hour in the presence of a parent, and it was established that 20 per cent of them definitely had a psychiatric disorder.
The most common type was depressive disorders, followed by anxiety disorders and attention deficit hyperactivity disorder.
Some 5 per cent of them had had significant thoughts of suicide in the past, and 4 per cent had engaged in suicidal behaviour. However, few were attending child and adolescent mental health services.
Prof Fitzgerald said depressive disorders were found to be equally common in boys and girls.
She said not all young people with psychiatric disorders needed to be referred to mental health services. Many could be helped through their difficulties by their families, friends and teachers, but those who needed to access the services should be able to do so quickly.
"This is not the case at present, with many clinics having long waiting lists for all but the most urgent cases. It is almost impossible at the present time to admit a young person who is seriously suicidal to an in-patient adolescent treatment unit, as the few such beds that exist are always full," she said.
"Health promotion programmes which encourage young people to look after their physical health are included in the curriculum in many schools.
"The results of this study suggest that we should be giving as much attention to mental health promotion programmes to provide true education for life," Prof Fitzgerald added.