Over a quarter of 13-year-olds in Ireland were heavier than the medical profession says they should have been when they were surveyed in 2012 - and 6% of the total were obese.
The latest report from the Growing Up in Ireland study also says that about 10% of those interviewed said they had been bullied in the previous three months, while 2% said they had bullied someone else.
Much of today's report is based on interviews conducted six years ago with over 7,400 13-year-olds and their families.
The research team was led by the Economic Social and Research Institute and Trinity College.
Most of the children were in good physical health.
But the report's co-author, ESRI Professor James Williams, said it highlights significant inequalities with socially disadvantaged children found to be at higher risk of poorer health, schooling and emotional and behavioural wellbeing.
Report highlights screen-time figures for children
For example, while over a fifth of parents in the highest social class reported some health problems, a third of parents in the lowest social class said their children were sometimes unwell.
Weight issues established in early childhood can be difficult but not impossible to reverse, the report says.
Over a quarter (26%) of the children were heavier than the medical profession says they should have been, with 6% of the total sample classified as obese.
Girls were significantly more likely than boys to be too heavy (30% compared with 24%).
Strong differences also emerged depending on family background, with 21% from the Professional/Managerial social class being heavier than they should be compared with 32% from homes which had never experienced employment.
The study discovered that 21% of those who were obese described themselves as being "just the right size" or "very or a bit skinny".
The report says girls were less likely to take part in physical exercise than boys, while poorer children were the least likely to participate in organised sport and physical activity.
Most of the 13-year-olds reported a high level of positive interactions with teachers, with 70% saying they were often or very often praised for their work.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
Overall, attitudes to school and teachers were positive: 29% of 13-year-olds liked school very much and a further 33% liked it quite a bit.
Girls liked school more than boys did, with 34% of children whose main caregiver had degree-level qualifications liking school very much, compared to 24% of those whose main caregiver had left school at Junior Certificate level.
The research found that 13-year-olds from one-parent families and from homes with lower educational backgrounds had poorer socio-emotional wellbeing.
In most cases parents had met all of their 13-year-old's friends. But about 10% of the children said they had been bullied in the previous three months while 2% said they had bullied someone else.
Boys and girls were about equally likely to have been bullied. The research says that the bullied children were at a greater risk of socio-emotional and behavioural problems.
16% said they had had an alcoholic drink - other than a few sips, with a higher rate among boys than girls (17% and 14% respectively).
Approximately 9% said that they had smoked. There was no difference in the levels reported by boys and girls. Levels were highest among those from lower social class and lower education backgrounds.
For both smoking and drinking the percentage reporting they were currently smoking and drinking was considerably lower than the percentage who reported having "ever smoked or taken an alcoholic drink".
The report's authors say this suggests that some of this activity may have been of an experimental rather than habitual nature.
Finally, slightly over 1% of 13-year-olds reported using cannabis while 3% had sniffed glue.