Number of Covid-19 cases among young people is worrying - chief medic
It has also been reported that the number of people who have died from the virus around the world has hit 500,000.
Dr Holohan said the number of people who were likely to travel from Ireland for non-essential trips and then return was greater than the total of tourists from outside the country coming in.
The Department of Health has also announced that there have been 24 new confirmed cases of the virus in the country.
He said: "We are starting to see a worrying trend, with the number of reported cases increasing, and some new clusters".
"We have been corresponding on an nearly weekly basis with the NTA and operating companies in relation to a number of measures, not least the compulsory wearing of face coverings".
The United States in total accounts for a quarter of positive tests, while Brazil has 1.3 million cases.
It is important to model the Covid-19 epidemic in Northern Ireland in order to plan the delivery of Health and Social Care services and to inform decision making with regard to social distancing and other restrictions.
The Chief Medical Officer has raised concerns over the return of global travel, saying it risks a second wave of the coronavirus here. As of midnight on Wednesday, June 24, the latest date for which county figures are available, Kildare had 1,478 cases.
New protocols will be in place at test centres to prevent the spread of Covid 19.
It comes on the day after the lowest amount of daily confirmed cases in June were recorded and brings to 25,462 the total amount of individuals to have tested positive. The risk of imported cases remains high. 1,462 people have now been discharged from hospital after recovering from the virus.
On Saturday the chief medical officer said the number of new coronavirus cases in younger people was a concern. "A further eight cases (35%) are in those aged between 35-54".
Dr Tony Holohan says it's an early indication, but it seems as though the level of infection is rising again here.