It has emerged that three tobacco companies have now threatened legal action over new plain packaging tobacco laws here.

The legislation was signed into law by President Higgins yesterday, and will take effect on a date yet to be decided.

The new laws - The Public Health (Standardised Packaging of Tobacco) Bill 2014 - were signed at Áras an Úachtaráin after the mandatory seven-day period.

May 2016 has been a deadline set for when it will be illegal to manufacture branded cigarettes - and after May 2017 it will then be illegal to sell them.

Japan Tobacco International, Philip Morris, and Imperial Tobacco have all written to the government through separate legal firms.

Ireland is only the second country in the world, after Australia, to make the move to plain packaging.

The Children's Minister James Reilly - who began work on the laws - told Newstalk Breakfast that the state is ready to stand its ground in court, if needed.