The Chair of NPHET's Irish Epidemiological Modelling Advisory Group Philip Nolan, says there is empirical evidence that settings involving crowds provide the platform for COVID-19 to spread.
The GAA has requested a meeting with the Acting Chief Medical Officer Ronan Glynn 'without delay', to present 'empirical evidence' which resulted yesterday in the Association's games and those of all other sports being placed behind closed doors.
There were up to 200 people allowed at sporting matches, but crowds will not be present now until September 13th at the earliest.
Training will also be limited to groups of 15 outdoors.
Nolan says that NPHET can back up their reasoning for advising the Government to take stricter measures to slow the transmission of the virus.
"Absolutely there is empirical evidence both here and internationally that the source of congregations that we know are occurring (so there is no issue here about sporting activity and training). If you bring people together you get two problems; a relaxation of physical distancing at the event and then congregation and recreation before and after the event," he said.
Nolan says people are meeting, congregating and travelling in cars together around sporting events.
"All of those things are happening, we know those things are happening. There is data on those things and there's data from outbreaks in this country and elsewhere that those settings are - unfortunately, because they are very enjoyable, the ideal opportunity for the virus to transmit. "
GAA inter-county training is scheduled to begin on September 14th, with the Championship slated to start at the end of October.