It's "disgraceful" that people are coming into Ireland from some of the US states worst hit by the coronavirus pandemic, according to Prof Luke O'Neill.
He said it's a "surefire way to increase the chances of spikes and surges" in this country.
On Friday, Newstalk's Barry Whyte spoke to some of those arriving here from the US - with one man arriving here from Dallas, Texas suggesting the entire coronavirus crisis was "overblown".
Speaking on The Pat Kenny Show, Prof O'Neill - professor of biochemistry at Trinity College Dublin - said the situation is "outrageous" and "chaotic".
He said: "Texas, Arizona for example... the numbers are terrifying there, and they're breaking records every day. And yet people are coming in to Ireland from those places - I think it's disgraceful.
"We're trying to avoid a surge or a spike, and yet people are coming in from a country that's got a very high level of virus in the community.
"This is a surefire way to increase the chances of spikes and surges coming to Ireland."
He acknowledged that travel is "obviously a complicated thing to control this", but argued that we should be doing "everything in our power" to control people coming into Ireland at this stage of the pandemic.
He observed: "The risk now absolutely is travel. The virus is almost gone - the numbers are well down in Ireland."
— NewstalkFM (@NewstalkFM) July 13, 2020
Elsewhere, Prof O'Neill said the Government should leave the full reopening of pubs until "a bit longer".
He noted that "study after study" shows pubs are the riskiest place to be.
He explained: "It's as bad as a meat packing factory, a pub... even in Australia this morning they've reported another cluster centred on a pub.
"It's a scientific fact that pubs are very high-risk environments, mainly because of alcohol. Even in places they were trying to socially distance in the USA, there was still a spike from a pub. Alcohol and social distancing are mutually exclusive things for definite."
Prof O'Neill said that "pubs are just very dangerous places to be", and that measures such as social distancing and good ventilation need to be fully in place.
He told Pat: "In my opinion the best thing we do now is we sit this round out, in a sense, and be very careful with these pubs.
"There's no doubt that if they open there will be an increase in cases - the question is how big that increase will be, and how we manage to handle those increases.
"Leave it a bit longer, and let's see what happens."