People caught crossing the Border from the North without an essential reason will be issued a €100 fine from this morning.
The new measure is part of the ongoing effort to clamp down on non-essential travel to suppress the spread of Covid-19.
Every adult in a car will be subject to the charge, not just the driver.
Over 3,500 people were fined in the Republic for non-essential travel, according to figures up to the end of last week.
Damien McGinnity from Border Communities Against Brexit says the new fines are needed because of the lack of cross-border co-operation on the pandemic.
"We think it's a bit incredible that political leaders can't get together and agree a strategy to test and quarantine everyone coming onto the island.
"We have over 5,000 death on this island - New Zealand have 25 dead. There's just no comparison," Mr McGinnity said.
This comes as the Government continues to tighten travel restrictions for both domestic and international travellers, with mandatory quarantine expected to be operational by mid-February according to the Taoiseach.
Last months, figures from the Department of Transport showed the majority of passengers passing through Dublin Airport did not have an essential reason for their journey according to the passenger locator forms they supplied.
Approximately 110,930 passengers arrived in the airport during the first month of the year, the majority of whom were travelling from Poland, France, the UK, and the Netherlands. It is thought many of the passengers were returning to Ireland having spent Christmas elsewhere.
Since January 16th, all arrivals into Ireland have been required to supply a 'non-detected' PCR test result on arrival, however almost 64,000 passengers arrived in the State before that date.