There have been 4,799 further confirmed cases of COVID-19 in Ireland.
As of 8.00am on Monday, there were 467 patients hospitalised with the virus - of which 104 were in ICU.
The five-day moving average of cases has passed 5,000.
Chief Medical Officer Dr Tony Holohan says there has been a sharp increase in cases among one age group.
"The level of disease and positivity rate amongst young adults has increased sharply in the last week.
"One in four people in the 16 to 34-year-old age group that are going for a PCR test have received a 'detected' result.
"This is one of the highest rates since the beginning of the pandemic".
Dr Holohan says this is a high risk for all those who come into contact with these people.
"This high level of disease in young people represents a high risk to those they come into contact with who have yet to be vaccinated or receive their booster.
"Therefore, if you have not yet received your booster you are best to avoid unnecessary contact with people outside your household."
'Only half isolating with symptoms'
He says people limiting their contacts to small numbers is key.
"Christmas week is an important time for many of us.
"However, it is also a time that presents unique opportunities for COVID to spread because of the possible extent of inter-household and inter-generational mixing.
"Each of us can take actions this week to protect ourselves and our loved ones, even if this means re-thinking plans for the Christmas period, especially if you are not yet boosted or vaccinated.
"As difficult as it may seem, limiting as much as possible your Christmas to small numbers and very close family will protect them.
"This is an exceptional sacrifice to ask after the very difficult year all of us have had, so please remember that it may be the decision that protects yourself or a loved one from the severe impacts of COVID-19."
And he adds that 'only half' of people with symptoms are isolating as required.
"It's also a concern that our latest Amarach tracking data shows that only half of people with symptoms are isolating.
"This is the single most important piece of basic public health advice - it is vital that you isolate as soon as you experience any symptoms of COVID-19 and arrange a PCR test.
"Do not meet up with others and put them at risk of COVID-19 or another respiratory infection.
"Do not rely on a negative antigen test as a basis for not isolating."