President Michael D Higgins has signed into law the legislation introducing plain packaging on cigarette packs.
The Public Health (Standardised Packaging of Tobacco) Act 2015 was approved by the Dáil and the Seanad last week.
Under the legislation, companies will be banned from producing branded tobacco products for the Irish market from May 2016 and banned from selling branded tobacco products in Ireland from May 2017. Australia introduced a similar ban in 2012.
Tobacco companies claim plain packaging infringes international law, could threaten future foreign direct investment and sets a dangerous precedent for other sectors.
The world's biggest cigarette firm is threatening to sue the State over the new law.
Philip Morris International (PMI), whose annual revenue of US$80bn is bigger than Ireland's tax take, says it will sue for damages if the law proceeds - forcing it to incur manufacturing costs - and an EU test case on plain packaging succeeds.
The US multinational makes the 'Marlboro' brand and has taken legal action elsewhere.