People arriving in Ireland from Britain who are not fully vaccinated will have to self-isolate for ten days in changes that come into force with immediate effect.
They will have to receive a negative PCR test on day five and day ten before they can exit the period of quarantine.
Those who are vaccinated will still have to quarantine at a stated address for five days, but can resume normal activity after that period if they get a clear PCR test.
Cabinet sources described it as a bridge to 19 July when a reopening of international travel is set to resume.
The Department of Health has also restated that there remains a legal requirement to self-quarantine if your journey originated in Britain even if you enter the State via Northern Ireland.
In a statement confirming the move this evening, Minister for Health Stephen Donnelly said variants of concern "continue to pose significant risks to public health and there is a specific concern around the Delta variant".
He said: "Non-essential travel should continue to be avoided, however, if you must travel to Ireland from Great Britain then you should follow the public health advice relating to home quarantine and you are advised to avail of the free day 5 and day 10 PCR testing.
"Last week, I launched a booking portal that allows anyone travelling to Ireland book a PCR test for no less than 5 days after arrival to Ireland. Those travelling from Great Britain who have not been fully vaccinated, are now advised to arrange a test for day 5 and for day 10. If you receive a non-detected result from both your day 5 and day 10 tests it is safe to cease your home quarantine."
Taoiseach Micheál Martin said earlier that the Government is continuing to monitor the Delta variant of Covid-19.
British Prime Minister Boris Johnson has delayed the easing of restrictions in England amid concern over the spread of the Delta variant, which was first identified in India.
Experts in Britain believe the variant is driving a rapid acceleration in case rates and hospital numbers and it is believed to now account for 90% of Covid cases there.
The Taoiseach said: "The broader picture is that the variant has increased in prevalence in the UK.
"I think it makes sense that we continue to monitor this, which we will take public health advice as we move along, but so far the reopening has gone well in Ireland."
Earlier, Minister for Transport Eamon Ryan said that the aim of tighter restrictions is to hold back the Delta variant for as long as possible while vaccination continues to increase.
Minister for Transport @EamonRyan says the Government is considering a change to rules for people arriving in Ireland from the UK, which would see a requirement to self-quarantine for ten days rather than five, with two negative PCR tests required | https://t.co/qwyuINUYaI pic.twitter.com/xB1ABMJ06s— RTÉ News (@rtenews) June 15, 2021
Meanwhile, Minister for Higher Education Simon Harris has called on public health experts to review the situation whereby fully vaccinated people still have to self-isolate after international travel.
Mr Harris said he does not believe this is logical and he thinks it should be re-examined very soon.
The minister said there was now a race between variant and vaccine and the tightening of travel restrictions from Britain agreed by Cabinet today was erring on the side of caution.
He added that there was no current public health advice to suggest there is any obstacle to the further easing of restrictions here in July.
The minister was speaking following Government approval of a plan to get third-level students back on campus in September.
He said the year of college on Zoom was coming to an end and students would soon be able to return to college, have a pint and who knows even make new friends.
A decision on the holding of large lectures will be taken by Government in July.
Researchers in the UK say the most commonly reported symptoms linked to the Delta variant there are headache, sore throat, and a runny nose.
Professor of genetic epidemiology at King's College London Tim Spector, lead researcher with the Zoe Covid symptom study, said the Delta variant feels like a "bad cold", especially for younger people.
Health Service Executive advice is that the main symptoms of Covid-19 are a new cough, high temperature, and loss or change in smell or taste.
An Assistant Professor of Virology at UCD said if the Delta variant took off the same way in Ireland, as it has in the UK, case numbers could begin to rise again as restrictions further ease next month.
Dr Gerald Barry said delaying the easing of restrictions for a few more weeks would dramatically increase the number of people protected against Covid-19 and allow the country to open up in a safer way at the end of July or beginning of August.
Speaking on RTÉ's Today with Philip Boucher-Hayes, Dr Barry said he would be cautious about easing restrictions too quickly.
He said: "The percentage of the population that have two shots of a vaccine is relatively low compared to the UK. They have about 75% of the population with one shot of vaccine, and over 50% of the population with two shots, and they're seeing a dramatic rise in cases with the Delta variant and an increase in hospitalisations."
He said people who are over 60 and have received one shot of a Covid-19 vaccine should be prioritised to get a second dose as quickly as possible.
Minister for Justice Heather Humphreys, meanwhile, said she has today lifted the visa requirements introduced in January that limited people entering Ireland from South Africa and South America.
She told RTÉ's News at One that those countries will return to the previous status quo, but that all countries on this list are subject to mandatory hotel quarantine.
She said the changes would open up travel for essential reasons from these countries, including for business, medical or family reasons.
The decision means that possession of a valid Irish visa is no longer required when arriving into the State from Argentina, Bolivia, Brazil, Chile, Guyana, Paraguay, South Africa, and Uruguay.
For people arriving from Colombia, Ecuador, Peru, and Suriname, the minister's decision means they no longer need transit visas to travel to another State via Ireland, but would require a visa to visit Ireland.
However, public health measures still remain in place for all those arriving into Ireland, and the Government strongly advises against unnecessary international travel to Ireland.