The head of the INTO says the re-opening of schools will see one million people going back into the education system.
A plan to re-open schools by end of August is expected to be announced later on Monday.
Details were discussed by the Government over the weekend.
Enhanced cleaning regimes, investment in school buildings and more substitute teachers are just some of the measures planned.
John Boyle, INTO general-secretary, told The Pat Kenny Show the operation is 'bigger than Normandy'.
"There's actually a million people going back into the school system in five weeks time from today - and that's not even counting probably a million parents that are going to be accompanying them as well.
"As you see on the streets of Dublin there are very few people out and about at the moment.
"And suddenly we're going to have this massive increase in public transport and lots of people congregating - and that's the big concern for us in all the talks.
"We've been really worried about that very first day of September, or late August, when everybody goes back, and to make sure that we get off to the right start to have it safe for staff, safe for pupils and safe for their parents and for the whole community".
"It's a big undertaking...we were getting homework from the department on a Friday evening at 8.45 and asking for the feedback then by Monday at noon.
"We have worked really, really hard with all the stakeholders over the last number of weeks to make sure that the plan today is strong".
"We'll see the plan today, but I'm happy with the detail in the plan - there's probably about 120 pages for the primary sector alone.
"Each principal and board of management will have to read that plan and put a lot of practice in place throughout this month now between risk assessments and hiring more staff to make sure absences are going to be covered."
"I suppose really the big litmus test with this is going to be maybe after a week or two when we're back: will that €1.70 per person per day be sufficient expenditure to make sure that we keep the schools open?
"I do have a concern about that.
"There's big talk about 300m - when you break it down, it works [out at] about €1.70 a day - is that going to be enough to cover all teacher absences, teachers are going to be mandated to stay at home, is it going to be enough to make sure that schools are cleaned to the level that's needed?"
"They're entitled to be safe... the key point here is that we want to stop the spread of infection in society, and if a million people go back into the school sector in September there's a big risk that we'll have a second spike - so obviously we have to represent our workers to make sure that they're safe and that we avoid that terrible, appalling vista for our country".