The long-awaited and controversial smoking ban in workplaces in the Irish Republic came into force at midnight.
Smokers enjoyed their last puff on Sunday before the midnight ban
If customers are caught smoking in pubs, restaurants and other enclosed workplaces, proprietors will now face fines of up to 3,000 euros (£2,000).
The new laws - described by Taoiseach Bertie Ahern as "landmark legislation" - are the toughest in Europe.
While health campaigners are delighted, publicans fear it will damage custom and question whether it is enforceable.
Some bars in the capital Dublin have set up wall-mounted ash-trays and heated outdoor areas to enable smokers to circumvent the ban.
Pubs in Northern Ireland are hoping for a cash bonanza, with smokers willing to cross the border to continue smoking a cigarette with their pint.
But supporters of the ban say that far from putting people off, smoke-free pubs and bars will attract a whole new clientele.
Mr Ahern said a recent survey suggested nearly three in four people who visited a bar in the last two weeks were non-smokers.
The survey also found that almost 60% of smokers would not be deterred from going out for a drink because of the ban.
"Being in a room in which there are smokers means being exposed to at least 50 agents known to cause cancer and other chemicals that increase blood pressure, damage the lungs and cause abnormal kidney function," he said.
But many Irish publicans are strongly critical, saying it will be impossible to enforce, especially on busy weekend nights or in areas where having a cigarette with a pint is a way of life.
"There are pubs, particularly in rural Ireland, where you have old folk coming in who have been sitting in the same seats for 30, 40 or even 50 years," said Con Dennehy, whose family runs a pub in Cork, south-western Ireland.
"It's going to be very difficult for publicans to ask those people who have been so loyal to them over the years to desist from smoking."
Sheila McKinnon, of the national executive of the Vintners' Association, which represents pub landlords and landladies, says the ban could stop tourists, including fishing enthusiasts, visiting Ireland.
She told BBC Radio 4's Today programme: "They could say to themselves, 'It's really expensive to come here to fish' and they may say 'To hell with this, we're sitting down having a pint after our dinner on a relaxing week's holiday but we can't smoke'.
"And they could then say, 'I might not bother coming back to Ireland next year'."
Ireland's junior health minister, Ivor Callely, said the ban would be difficult to implement to start off with because of the "cultural change" and because people found it difficult to give up smoking.
But he told the Today programme it could ultimately result in more people visiting pubs, attracted by the smoke-free atmosphere.
He said: "We are known as a beautiful island nation with great pubs. It's not smoke-filled rooms people come for, it's to come for the craic, to come for the Irish music and to come for the friendly people.
"I think that will continue and, if anything, the ban will encourage more people to enjoy the sessions that do go down in Irish pubs."
The ban will not apply in hotel rooms, prisons, nursing homes and psychiatric hospitals.
Meanwhile, cancer charity workers and a group of doctors in Northern Ireland are calling for a similar ban.
In a letter to the media, they said people north of the border deserved the same protection from passive smoking.