Passengers arriving into Ireland from the US, Canada and France now face 12 nights of mandatory quarantine in a hotel.
European countries like Italy, Belgium, Luxembourg and Austria are also on the updated list.
It came into force at 4am this morning – despite the fact that the system is currently fully booked.
On Monday, the Government ‘temporarily suspended’ bookings due to a lack of available rooms.
It has blamed the overload on an "unacceptable" number of passengers arriving into the country without booking ahead.
On The Hard Shoulder last night, the Health Minister Stephen Donnelly insisted the problem was not a lack of capacity – and blamed airlines that been letting people board planes without pre-booking quarantine rooms.
He said capacity in the system will increase by just over 300 rooms on Monday, before increasing by more than 300 again the following week.
The Government is also considering making people who have been fully vaccinated exempt from mandatory quarantine.
This morning, UCD Associate Professor Julien Mercille – a member of the Independent Scientific Advocacy Group, which advocates for Zero Covid – said increasing the countries on the list is a positive step.
“If you add more countries and more countries at least what you are doing is you are slowing down the importation of cases and variants,” he said.
“Especially the variants. If you have new variants coming in, in greater numbers, then it could really spread like wildfire.”
Meanwhile, figures show that the number of people arriving at Dublin Airport increased slightly last week after the original mandatory quarantine list of countries was enforced.
Just over 14,000 people arrived at the airport from Monday to Sunday last week – an increase of one-fifth on the week before.
The figures also show that a larger share of the arrivals were non-residents.
Nearly 4,900 of the passengers arriving at the airport said they were returning home to Ireland.
Over 9,500 were people who do not live here full-time – up from 8,500 the week before.
During the week, Newstalk reporter Henry McKean travelled to Dublin Airport and met with passengers who had brought forward their flights to beat today’s quarantine deadline.
Yesterday, meanwhile, a flight into Dublin from Paris this morning was “as good as full” with people looking to beat the deadline.