People from Northern Ireland who cross the border without a reasonable excuse will face a €100 fine from this morning.
It applies to anyone who isn't 'ordinarily resident' in the Republic, including people living in the North.
Every adult in a car will be subject to the fine, not just the driver.
Damien McGinnity from Border Communities Against Brexit says the new fines are needed, due to the lack of cross-border co-operation on the pandemic.
He said: "This island has a lot of water running around it - a natural boundary to COVID coming onto the island.
"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 dead on this island - New Zealand has 25 dead. There's just no comparison."
Figures released last week show that over 3,500 people were fined in the Republic for non-essential travel, according to figures up to the end of last week.
Fines of €500 can be issued for anyone travelling to ports or airports without an essential reason, with more than 350 people fined on their way to or from Dublin Airport over just a week.
However, there hasn't been a substantial change to passenger numbers at the airport since fines were introduced.
As of last week, gardaí also now have the power to carry out random spot checks on anyone who has arrived into the country to ensure they are staying at home.
Anyone found to be not complying with the rules could be fined up to €2,500 and be jailed for up to six months.
