A more severe regime of restrictions on retail, travel and sport have come into force as part of Government measures aimed at bringing the spread of coronavirus under control.
All non-essential retail will close from 6pm this evening for at least a month, while gyms and swimming pools are also to close.
No social gatherings, including New Year's Eve celebrations, are allowed from today.
All household visits are also banned and there will be a 5km limit on travel.
The Government has also decided to extend the ban on travel from the UK to January 6.
Announcing the new measures yesterday, Taoiseach Micheál Martin said that Covid-19 infection rates were growing "exponentially".
"We must return to full-scale level 5 restrictions for a period of at least one month," Mr Martin said in a televised address, calling the situation "extremely serious".
Justice Minister Helen McEntee said that in a short space of time, the spread of Covid-19 has "surpassed the worst case scenario".
Defending the Government's handling of the situation, she said decisions that have been made in recent weeks were all based on a number of factors.
Speaking on Morning Ireland, Ms McEntee said the Government had looked at everything from the best case scenario over Christmas to the worst.
However, she said, that she is not sure anyone would have predicted where we are in the last two weeks.
She said January will be a difficult month as we move back to the full Level 5 restrictions.
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"January can be a difficult month and we are trying to put in measures to bring down this sharp rise," she stated.
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The Minister also said the new restrictions will "lay the ground for" the vaccine which will be rolled out over the next few months.
She said supports will remain in place for those sectors and workers who are impacted by the latest pandemic restrictions.
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Business group Ibec said the Government decision to implement stricter Level 5 Covid restrictions is devastating for the many businesses and service providers who will be out of employment once more as we enter the New Year.
Retail Ireland, the Ibec group that represents the retail sector, said the move to close non-essential retailers came as a significant blow to the many businesses that would now have to close.
Retail Ireland's Arnold Dillon said that while retail has demonstrated it is a safe setting for customers and staff, there is now clearly an aim to dramatically reduce all movement across society.
"This is obviously very disappointing to the many thousands of businesses that had been planning to stay open and trading into the New Year," he said.
"Now that the decision is made, it is vital that these measures work, and we are soon in a position to reopen again. The rapid rollout of the vaccine is now essential to avoid prolonged disruption and restrictions over the coming months," he added.
Retail Excellence also expressed its "shock and dismay" at the Government decision to extend new Covid-19 restrictions to include non-essential retailers, adding that the sector would need urgent financial support.
