A ban on the sale of packs of ten cigarettes came into effect from midnight last night, although there will be a period of grace for retailers to get rid of stocks.
Confectioneries that resemble tobacco products are also banned.
The Office of Tobacco Control says it hopes the measure will discourage young people from smoking.
But the smokers' rights organisation, Smokers Against Discrimination, says the ban will result in people of all ages smoking more.
However, research published last year showed that 76% of young people who smoke buy the pack of ten.
Up to 77% of these say they would be likely to quit if cigarette prices doubled.
One of the world's leading experts on tobacco, who is speaking at a conference in Dublin today about smoking habits, says cigarette manufacturers deliberately use the ten-pack to encourage younger people to smoke.
Professor Kenneth Warner says the Irish tobacco industry needs 50 children to start smoking every day to replace the numbers who quit smoking or die from smoking every year.
Any retailer who breaches the ban faces a fine of up to €3,000 or up to three months in prison.