Health Minister Leo Varadkar has admitted that "dozens" of criminal cases could be affected by the loophole in the country's drug laws.
Possession of ecstasy, magic mushrooms, so-called 'head shop' drugs and other new psychoactive drugs will be legal until later today following a decision by the Court of Appeal yesterday morning.
The Government was forced to rush through emergency legislation in the Dáil last night after the court struck down a law banning so-called legal highs.
A Government regulation declaring illegal a psychoactive substance sold lawfully in 'Head Shops' until 2011 was found to be unconstitutional.
In what the Court of Appeal said was a "constitutional issue of far-reaching importance", the three-judge court unanimously said a regulation making the possession of methylethcathinone illegal was invalid.
Reacting to the judgment, Mr Varadkar said that he has been advised that the number of cases involved is "relatively small".
The Irish Independent has learned that convictions already handed out are not open to being overturned, and the impact of the court's decision is limited to cases "which are currently in process".
Mr Varadkar said that on foot of the judgment, possession of the 70 types of drugs is not illegal, but the "sale, supply, import and export" of such substances remains against the law.
Passage of the bill through the Dáil last night and the Seanad today will close the loophole that has been opened up in the law.
Mr Varadkar, a qualified doctor, urged people tempted to take such drugs to consider the harmful impact that they may have.
He said that once it was realised the case was heading for the Court of Appeal, he and his officials began drafting the emergency legislation, which he said was approved by Cabinet in January.
Until the new emergency legislation is passed, possessing a range of substances including ecstasy, benzodiazepines and new psychoactive substances, so-called 'headshop drugs' is legal.
"All substances controlled by means of Government Orders made under section 2(2) cease to be controlled with immediate effect, and their possession ceases to be an offence," states the memorandum explaining the emergency bill.
According to the document, the judgment has no implications for approximately 125 substances, including cannabis, heroin and cocaine, as they are outlawed by another area of the bill.
The court said Section 2(2) of the 1977 Misuse of Drugs Act, under which the regulation was brought in, was unconstitutional because it purports to vest the Government with law-making powers which are in the exclusive authority of the Oireachtas.
The State indicated today it may seek to appeal the decision to the Supreme Court on a point of exceptional public importance.
The bill will go the Seanad this morning before being sent to President Michael D Higgins for his signature.
'LEGAL HIGHS' THAT WERE TAKEN OFF OUR SHELVES
Before the ban on 'legal highs' in 2010, these were some of the most popular.
Mephedrone ('Meow Meow'): Often sold as pills or powder, it has a similar effect to ecstasy, amphetamines and cocaine.
LSA: An alternative to LSD, sold in tablet and seed form, it causes hallucinations.
Piperazines ('Bath Salts'): This can cause arousal and euphoria.
Spice ('Incense'): Involves the spraying of synthetic cannabis onto dried plant mixtures, with the result being sold as a herbal high.
Kratom: This common alternative to heroin is taken in tea.