Parents doing school runs should refuse to bring another child if their car is already full, the Automobile Association (AA) has warned.
The AA said there was a high element of risk when too many children were transported in an overcrowded car.
In the event of an accident the outcome for any child travelling without a seat belt can be catastrophic.
A survey conducted by the AA showed a significant minority of parents were willing to offer a list to the child of a friend or neighbour, even if the car was already full.
A total of 16pc of parents admitted to driving, on certain occasions, when they had more passengers than the car was designed to carry.
However, the survey also showed there was now greater awareness of the risks involved, compared to some years ago.
Social pressure on drivers to transport extra passengers, both adults and children, had eased in recent times.
The AA also pointed out that in 2013 one in five of those who lost their lives on Irish roads were not wearing a seat belt.
It said it strongly supported the recent increase in penalty points, from two to three, for those who failed to "belt up."
It also reminded drivers that they were responsible for ensuring all their passengers aged under 17 were wearing a seat belt.
"Even if a friend is stuck, and can't get to the school gates on time, don't be tempted to stack the kids into the back," said Conor Faughnan, the AA's Director of Consumer Affairs.
"Not only are you risking the life of the unrestrained child, but you're also endangering all the other passengers.
"If a crash occurs, an unbelted or unrestrained child will be literally catapulted forward, smashing into whatever they come into contact with first.
"It doesn't bear thinking about, and it's not something anybody would ever want to be responsible for."
He pointed out that when a crash occurs an unbelted passenger continues travelling at the same speed as the car in which they are in.
Therefore a passenger travelling in a car at 50kmh will be flung forward at this speed in the event of a crash. This is equal to a force of 3.5 tonnes.
"The heavier the person the more damage they will cause," said Mr Faughnan.
The AA also advised its insurance customers that failure to wear a seat belt could have financial consequences.