Professor Luke O'Neill says outdoor socialising is safe - but people need to spread out and avoid dense crowds.
The Trinity immunologist was speaking after images and videos of large crowds gathered in Dublin city centre emerged over the weekend.
Some officials - including Dr Tony Holohan - have said they were shocked by the crowd sizes.
However, there's also been criticism of city authorities for not providing more outdoor spaces and facilities for people in the capital.
Professor O'Neill told The Pat Kenny Show outdoor socialising is much safer than indoors - but it isn't perfect, especially when there are large crowds involved.
He said: "That problem in Dublin… if they spread out more it would have been fine. The density of the people is the issue there.
“You can understand people wanting to get out. In many ways, we saw this coming… and next weekend will be the same, watch, with the bank holiday.
“If people just spread out a bit, and aren’t in such dense crowds, it’s safe outdoors.
“I wouldn’t blame the young people - they want to get out and get back to things. But it’s a shame it wasn’t anticipated better.”
The Trinity professor said he's looked "very hard" to find examples of outdoor 'superspreader' events but hasn't really found them.
He said: "There were one or two [such events], but there was a period indoors during the event.
“The trouble is if the Indian variant was in that crowd… it’s much more transmissible. Maybe it will emerge with a new variant there is outdoor spread.
“The analogy is if you can smell someone’s perfume or smell someone smoking… then you could be getting exposed to the virus. But time is the other key variable… if they were in that massive crowd for an hour or two, then that would be a worry.
“There are so many variables, really. We have to wait and see. I’m hoping there won’t be any evidence now that was a superspreading event.”
Concerts and antigen testing
Elsewhere, Professor O'Neill highlighted the findings of a recent study published in The Lancet journal examining whether indoor concerts can resume safely.
He explained: “There was 1,047 people… they antigen tested them, and they were all negative. They split them into two groups: half [spent] hours indoors at a concert, and the other half went home.
“It turned out one person in each group was PCR positive - they were negative in the antigen test, but positive in the PCR.
“Eight days later, they tested them and two people in the control group had gotten infected… nobody in the concert was positive.
“What it tells us is [that with a] good antigen test on the day of an event… good ventilation at the venue… they wore face masks... and nobody got infected. They deemed the concert safe."
He added that the study "shows the power of antigen testing to protect people".
It comes ahead of plans to trial concerts in Ireland over the coming more, ahead of a return of indoor events later in the summer.
