Anyone arriving into the Republic of Ireland from overseas will now need proof of a negative Covid test.
The measure came into effect yesterday.
It must be a professionally taken antigen or PCR test, as the Government will not accept a self-test.
Meanwhile, passengers arriving in Northern Ireland from abroad will be required to take a pre-departure Covid test from 4 o’clock on Tuesday morning.
The test can be either a lateral flow or PCR.
The measures apply to anyone aged 12 and over, including people who have been doubly vaccinated.