He urged people to keep their contacts as “low as possible in order to protect those around you”.
His comments come as another 5,279 cases of Covid-19 were confirmed today.
As of this morning, there were 443 people in hospital with the virus, of whom 102 were in intensive care.
More than 1.6 million people have received a booster or third dose so far, with over 75,000 doses given yesterday - the most in one day.
Dr Holohan said: “This week, try and only meet with the people with whom you will spend Christmas Day.”
He said Omicron now accounts for approximately two-thirds of cases in Ireland now and public health officials are seeing more household contacts test positive as a result.
“If there is one positive test, whether from a PCR or an antigen test, within a group such as a household then there is a significant chance that others are already infected, even if not yet testing positive. If one person from a household tests positive prior to an event or gathering, then none should attend,” Dr Holohan said.
Meanwhile he urgent parents “to support the young people in your lives to reduce their contacts as we continue to see incidence rising in our young adult population”.
“I know that this is not an easy task for any of us and it can feel very unfair that we continue to ask so much of our young people, particularly at this special time of year.
“This group have made significant sacrifices in order to protect loved ones and I would like to thank them for that. It is important now that we continue to encourage and support each other to make sacrifices, take responsible actions and continue to follow the public health advice.
“As we roll out the booster programme, it is important to remember that approximately one in six young adults are yet to come forward for vaccination or are not fully vaccinated. Vaccination offers very good protection against severe illness, so it is important that everyone who is eligible for a vaccine comes forward for one.
Meanwhile the HSE’s testing capacity is under huge strain due to the wave of Omicron currently spreading across Ireland.
There are test appointments available in just seven counties tomorrow with the other 19 being completely booked out for Wednesday.
There were no appointments at all in the East, South and Southeast with Clare, Donegal, Leitrim, Longford, Mayo, Roscommon and Sligo the only counties with capacity still remaining on Tuesday night.
Prof Philip Nolan of Nphet today highlighted an uptick in the number of cases being detected in the Dublin region along with an increase in incidence among the 16 to 34-year-old age group.
The HSE can currently administer approximately 225,000 PCR tests per week and over 33,300 tests were completed by the HSE on Monday, across hospitals and in the community, which is close to its maximum daily capacity.
Approximately 180,000 people have been tested by the HSE in the past seven days, with an average of 25,000 per day reporting to test centres in the community.