Ireland will require arrivals from Great Britain to provide evidence of a pre-travel negative Covid test result from tomorrow Saturday 9 January.
The new measures apply to arrivals from England, Wales and Scotland.
They will also apply to those whose journey originated in South Africa who until Friday night are banned from entering Ireland.
Arrivals will also be advised to self-isolate for 14 days upon their arrival in Ireland.
In a statement, Ireland’s Department for Foreign Affairs said: “From 9 January onwards, any passengers arriving into Ireland whose journey originated in Great Britain or South Africa are required to have a negative / not detected result from a pre-departure Covid-19 PCR test taken within 72 hours before their arrival in Ireland.
“These people will also be advised to self-isolate for 14 days upon arrival in Ireland.”
Ryanair issued a statement in response to warn passengers of the new rules, saying: “If you are travelling from the UK to Ireland you must be in receipt of a negative PCR Covid test result within 72 hours of departure.
“You are required to present evidence of your negative test at departure (email, text message, document).
“Upon arrival in Ireland, you are advised to self-isolate for 14 days and to take a further PCR test on or after five days following your arrival.
“Your failure to produce confirmation of a negative PCR test may result in prosecution on arrival into Ireland.”
Failure to produce the negative test result could mean a fine of up to €2,500 (£2,261) and/or imprisonment for up to six months.
Ireland has seen a surge in cases due to a loosening of restrictions in the run-up to Christmas and the arrival of the more contagious variant of the virus, which was first detected in the UK.
Of the positive cases that had arrived from Britain in December, 41.3% had been the new variant, said Prime Minister Micheal Martin.
The country is already in its top tier (Level 5) lockdown, but continuing record daily case numbers meant new restrictions could be likely.