The highest level of public health measures restricting people’s movements and social interactions came into effect at midnight.
Under Level 5 measures, people are asked to stay at home and to exercise within 5km from home.
Non-essential retail and personal services are to close, but schools and childcare remain open.
Level 5 measures will be in place for six weeks and Dr Holohan said everyone needs to stay at home, other than for essential reasons, and to follow the public health advice.
In a tweet last night, the Taoiseach said: “As we think about the next six weeks of increased restrictions at Level 5, we just need to remember that we are doing this to protect our families and the most vulnerable in our communities.
“We will come through it and we will see each other again.”
Gardaí have begun what they say is a major high visibility policing operation to support the new public health restrictions to curb the spread of Covid-19.