The ban, designed to deter young people from smoking, had been slated for last autumn - but was delayed because too many 1o-packs were still in the supply chain.
An Office of Tobacco Control survey indicated that 16% of Irish youngsters aged 12-17 smoked, spending up to €40 a week on cigarettes.
Almost 80% bought cigarettes in 10-packs. Most said they would "probably quit" if prices were doubled. Ninety-two per cent of the youngsters said they were never asked for proof of age.
Separate figures show a 5.3% increase in the numbers smoking in Ireland, despite the pub and workplace ban introduced more than two years ago. The increase is mainly attributed to an influx of eastern Europeans.