An HSE report said the man had mild symptoms after returning from holiday.
Ten households and a sports team became infected with Covid-19, when one man didn't restrict his movements after returning from holidays.
A HSE Mid-West report says the man had mild symptoms, including a runny nose and a sore throat, but his temperature was normal and he socialised with friends, before testing positive.
In total, his case led to 56 others contracting the virus.
Immunologist Professor Luke O'Neill says it shows just how quickly Covid-19 can spread.
''That's how contagious it is. It's a very very contagious virus, a single person infects 56 people, isn't that incredible'', he said.
''And that wouldn't be atypical, all the epidemiology is telling us one person seeds infection in 10, 20, 30 people.
''So you've got to keep in your mind, just imagine you're infected when you're out and and about, that will then restrict our behaviours.''
The government is expected to reveal greater restrictions sometime this evening, with most shops likely to close, along with gyms, while pubs and restaurants will be takeaway only.
Schools, creches and essential retail outlets will stay open under the new rules which are set to be a mix of Level 4 and Level 5.
