Anyone arriving into the Republic of Ireland from overseas will now need proof of a negative Covid test.
It must be a professionally taken antigen or PCR test, and came into effect yesterday.
Meanwhile, passengers arriving into Northern Ireland from abroad will be required to take a pre-departure Covid test from 4 o'clock tomorrow morning.
The test can be either a lateral flow or PCR.
The measure applies to anyone aged 12 and over, including people who have been double vaccinated.