NPHET's Professor Philip Nolan has explained why you can't gather in big groups in pubs, restaurants or homes.
Health chiefs have not seen an improvement in Covid-19 cases in Dublin yet as the capital faces an extension of restrictions.
Lockdown restrictions were due to lift on October 9.
But Mr Nolan has said that evidence from other countries shows that pubs and restaurants "drive community transmission".
One of the main considerations for health officials when deciding on new lockdowns is the 14-day incidence rate of confirmed cases per 100,000 population.
Ireland doesn't have Covid-19 under control says Nolan.
He posted on twitter: "The virus exploits the moments we come close as an opportunity to transmit.
"It’s spreading rapidly right now, between households, especially in young adults.
"The message: mix with one other household, keep your distance when socializing (even in your own home), get outdoors, limit your social contacts to those that are really important to you, stay home and seek a test if you feel unwell.
"What’s a virus between friends? Well, it sustains exponential growth of infection, and exposes our loved ones, and the vulnerable (young and old) to real danger. The special times, when we relax with those most important to us, are the very times we need to be vigilant.
"The virus is mindless, a piece of biological machinery that has evolved to use our sociability as a means to transmit and survive. We need to be mindful, and care for each other by keeping our distance when we are together. "
