The scenes of large crowds gathering over the weekend "looked like V-Day" celebrations, Dr Colm Henry says.
The HSE's Chief Clinical Officer says COVID-19 "doesn't surrender", and the pandemic is a "war" that still continues.
He was speaking after large crowds gathered in the capital on Saturday evening, including around the South William Street and Temple Bar areas.
Chief Medical Officer Dr Tony Holohan expressed shock over the scenes, suggesting it looked like a "major open-air" party.
On Newstalk Breakfast, Dr Henry said it's understandable why some people feel the need for "some kind of release" as restrictions ease.
He said: "Younger people for the most part did their best to protect their older citizens. A huge acknowledgement is required by all of us to them… they’ve suffered from isolation and lost opportunities in life.
“But some of the scenes looked like V-Day, as if the virus had surrendered. This virus doesn’t surrender.
“It exploits our weakest position - and our weakest position of course is those who are unvaccinated. It exploits congested gatherings, and certainly situations where there is alcohol."
“This is not V-Day… it’s a longer war.”
He acknowledged that outdoor gatherings are safer than indoors when it comes to COVID-19, but warned that congested crowds for long periods with alcohol involved “do not mix well”.
However, he said there’s still a “huge amount” to be optimistic about - including a significant collapse in COVID-19 case numbers, especially among vaccinated groups.
It’s expected that 50% of the adult population will have received at least one dose of a vaccine by the end of today.