Irish customs officers now have powers to confiscate cars if owners are carrying fireworks across the border.
The comment was made by Irish Justice Minister Michael McDowell to the Dail.
He also said people from the south were "queuing up" in Northern Ireland to buy fireworks but said he hoped they would be stopped on their way back.
Mr McDowell was responding to a question from Enda Kenny, the leader of the main opposition party, Fine Gael, about the importation of fireworks.
Mr McDowell said: "No legislation is needed on fireworks, but I echo (Mr Kenny's) remarks that the law is the law and must be respected.
"People are queuing up north of the border with the intention of bringing them south.
"If a customs officer stops them, which I hope happens, there will be a criminal prosecution and their cars are liable to be confiscated.
"I warn people that the law is tougher than it was and the consequences of breaking it are serious."
Labour's Joe Costello pointed out that the law on fireworks in Northern Ireland was the "exact opposite" of the law in the republic.
The tanaiste, Mr McDowell, said he would prefer an all-Ireland approach, but he was not yet in a position to legislate for Northern Ireland.
Last year, 86 people were injured by fireworks across Northern Ireland, mostly young men.
The dangers were highlighted last weekend, when a nine-year-old boy was treated in hospital after a firework went off in his hand in Belfast.
More than 1,000 illegal fireworks were also seized by detectives in County Tyrone.
The police said 1,000 bangers, 125 rockets and two boxes of fireworks were found during the search of a house at Evish Grove, Strabane, on Saturday.
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