There is "no question" that the government may have to introduce more restrictive measures if the number of COVID-19 cases continues to rise, according to Professor Kingston Mills.
The Professor of Experimental Immunology at Trinity College Dublin said the increase in people contracting the virus over the last week shows the trend "is worryingly upwards".
It comes after 45 new cases of coronavirus were confirmed in the Republic, bringing the total number here to 26,109.
The average number of cases over the last five days stands at 44, a figure which the Acting Chief Medical Officer Dr Ronan Glynn described as "clearly concerning".
Speaking on Newstalk Breakfast today, Professor Mills said: "If [the trend] continues to go upwards then there is no question that the government will have to look at either delaying Stage 4 or reintroducing more restrictive measures.
"There were over 200 cases in the last five days, compared with three weeks ago when we were down to around 100 a week, so there is a mini resurgence in the infection of the pandemic in Ireland.
He acknowledged that while several cases can be attributed to an outbreak at a pet food factory in Co Kildare, there are also clusters present around the country.
Professor Mills said: "It seems like some of these [clusters] are emanating from the fact that certain individuals are not isolating when they have symptoms.
"That's a significant concern because if people don't isolate when they're waiting to be tested or to confirm that they're positive then they're going to spread the virus."
This echoes a warning from GPs that there is an increase in the number of people with symptoms of COVID-19 who are not self-isolating.
Professor Mills also said the re-opening of pubs might have to be delayed if the number of coronavirus cases continues to rise.
Pubs that only serve alcohol are due to re-open on August 10th as part of Phase 4 of easing restrictions.
He said the opening of small rural pubs is not an issue, but larger pubs which cannot control social distancing may be a concern.
He added: "The indoor contacts of large groups are a real problem."
The wearing of masks would "really help" in preventing the spread of the virus, he said.
On whether some counties or regions may have to go into lockdown, Professor Mills said such a measure would be "difficult to implement".
"On a county basis, I think it would be too difficult to implement.
Professor Mills also stated it was important for young people to note that COVID-19 "is not a trivial infection".
Of the cases notified yesterday, 77% are under 45 years of age and 50% concern people aged between 25 and 44.
He said: "The lasting effects of this infection, even in young people, is very, very dramatic, it can go on for months after in certain people.
"It really is a very serious illness for all age groups, now there are more older people that will die from it than younger but younger people can still get infected and they're the big concern now."