Drivers will not receive penalty points if they do not have enough seatbelts to restrain all their passengers, according to the Department of Transport.
Since yesterday motorists have been liable for two penalty points if they or passengers under 17 are not correctly restrained. This increases to four points if the courts convict a driver.
Parents on school runs often carry four children in the back of a car, but most standard saloons have just three seatbelts.
A spokeswoman for the Department said the new law applied to the use of seatbelts in a car. If the seatbelts did not exist, then the driver could not be penalised for not using them.
She said common sense must prevail and, if there was a shortage of seatbelts, then the youngest passengers should be strapped in first.
Children under four should not be seated in the front of the car, unless they were suitably restrained, she said. Some safety groups recommend that children under 12 are not seated in the front of a car with air bags.
Manufacturers and safety groups stress that babies in rear-facing seats should never be placed in the front seat of a car if an air bag is present.
Preliminary figures for the number of people who received penalty points for seatbelt offences yesterday were not available last night. However, a Garda spokesman, Supt John Farrelly, said he expected that some penalty points would have already been issued.
He said the need for seatbelts had been well publicised, but many people still did not comply with the requirements. Gardaí made almost 55,000 detections for seatbelt offences last year.
Mr Pat Costello, National Safety Council chief executive, said he continued to be astonished at the number of parents who allowed their children to travel unrestrained in cars.
Only 20 per cent of back-seat passengers use seatbelts, while 72 per cent of front-seat passengers use the restraints.
Supt Farrelly warned that gardaí would only exercise their discretion in giving penalty points for seatbelt offences in "extreme situations or extenuating circumstances".
He said gardaí were unwilling to make exemptions as certain people would seek to exploit this.
Drivers from Northern Ireland and other jurisdictions are issued with on-the-spot fines for speeding and other traffic offences, but only holders of Irish licences can receive penalty points.
Meanwhile, the mandatory provision of seatbelts in school buses has been examined by the Departments of Transport and Education.
A Transport spokeswoman said a lengthy consultation process had just concluded its work.
