The release of the Leaving Cert results and CAO offers will not happen until early September - and further delay entry to college for first years.
The two key events in the annual school-leavers' calendar look set to run at least three weeks later than usual because of havoc wrought by Covid-19.
Education Minister Norma Foley has announced that calculated grades will be released on September 7, with the first round of CAO offers to come on September 11.
More than 60,000 students are now awaiting their results under the calculated grades process that replaced the June written exams this year.
About four in five of those have applied for a college place through the centralised CAO application system.
Because of the disruption to education caused by coronavirus, third-level colleges have already pencilled in later reopening dates.
But due to the latest delay, these dates may have to be pushed back further.
Sinn Fein education spokesperson Donnchadh O Laoghaire said the announcement yesterday evening that this year's Leaving Cert results will not be released until September 7 is a massive blow to students and their families.
"This is a hugely disappointing turn of events for students and it adds to the uncertainty, stress and anxiety they have had to face during the Covid-19 pandemic.
"Not only that, it raises serious questions about how the new third level year will work and when it will commence.
"We need real clarity now on how this announcement is going to impact on the beginning of the new third level year and what this means for students who have applied for third level places in other jurisdictions; many of whom have been offered a course and are awaiting their results to secure their place.
"We also need clarity on what this will mean for the results appeals process."
He added: "All of these questions need to be answered without delay.
"This is not the fault of schools and teachers, who have held up their end of the bargain and submitted calculated grades on time."
The Department of Education has said that all students, whether they opted to receive the calculated grades or not, will have the option of taking the Leaving Certificate examination later in the year should they wish to do so.
Students unhappy with their calculated grade can appeal, with this process open from September 14.