The proportion of the Irish population infected with Covid-19 is three times higher than that detected through other surveillance methods, the HSE said.
Almost 60,000 people aged under 70 contracted the disease up to mid-July, representative sampling of antibody levels in Dublin and Sligo suggested.
The estimated prevalence rate is 1.7%.
Medical chiefs have expressed concern socialising before and after sporting events could be contributing to coronavirus spread.
Principal investigator Dr Derval Igoe said: “It is not surprising that a relatively low national seroprevalence of 1.7% was observed here.
“Other countries in Europe, such as Spain and Italy, where there has been a much more intense epidemic, have reported national seroprevalence estimates of 5% and 2.5% respectively.
“This means that the vast majority of people living in Ireland had not been infected with Sars-CoV-2 virus by the time of the study.”
Of those who were found to have antibodies, 73% had symptoms that are included in the Irish Covid-19 case definition; that is, one or more of the following: fever, cough, shortness of breath, loss of sense of taste or loss of sense of smell.
One third of all those who were found to have antibodies reported loss of sense of smell and/or taste, the study showed.
The HSE added: “Using the prevalence data from this study, the Health Protection Surveillance Centre estimates that that 59,500 people in Ireland in the age group 12 to 69 years had been infected with Sars-CoV-2 up to mid-July; three times more than that detected via surveillance of notified cases.”
On Thursday the National Public Health Emergency Team said another 136 cases had been detected and one death recorded.
Socialising before and after sporting events is of concern, Ireland’s acting chief medical officer said.
Dr Ronan Glynn has faced challenges from the GAA over how restrictions are handled.
He said: “We have had cases and clusters linked to socialisation specifically related to sporting events.
“The measures are more about decreasing congregation in the round.
“It is not about targeting particular sports or particular organisations.”
Professor Philip Nolan is modelling spread of the disease for the Government.
He said: “The overall picture is of a sustained high level of new cases being confirmed per day, also a shift in the pattern of those cases.”
He said the number of possible cases of community transmission was increasing and recent restrictions were designed to bring outbreaks under control.
The R-number is now at or above 1.2.
Prof Nolan added: “There are two concerns now, the number of new cases per day remains high and the pattern has changed from large outbreaks in specific settings to much smaller outbreaks widely distributed across the country.
“The measures announced this week, asking us to stay apart, aim to suppress Covid-19 in the community.”
Dr Glynn said health system workers were exhausted, schools were planning to reopen, nursing homes should be protected and the economy needed to recover.
“The measures we took this week, while not palatable or nice, we believe, given the rising number of cases across the country, are proportionate.
“We are asking everyone to make a small sacrifice.”