38% of homeless children have significant mental health or behavioural disorders.
That is according to a new study by the Royal College of Physicians of Ireland.
It is calling for the government to act now to prevent so many children experiencing homelessness or inadequate housing.
Dr Julie Heslin, the lead author, says this causes a serious impact on children's health
"Research would seem that the type of illness that they experience includes depression anxiety, behavioural problems," said Dr Heslin.
"And then for the younger child, occasionally, developmental delay.
"It impacts on younger children but also up to adolescents."
Meanwhile, St Vincent de Paul said it is expecting more than 60,000 calls for help this winter from families in need.
The charity is launching its annual appeal today ahead of what it says is its busiest time of the year.
It says this year's focus is on helping children, particularly the almost 4,000 in emergency accommodation around the country.