This year's Leaving Certificate results will be published on 7 September, three weeks later than usual, the Minister for Education Norma Foley has confirmed.
CAO offers will be made four days later on 11 September.
On Monday a student portal will be opened to allow students register to receive Calculated Grades.
According to a spokesperson, the delay has been caused by "what is a brand new process and the data has to be gone through".
There are 450,000 grades to be processed and checked.
61,000 have applied to receive Calculated Grades under the process.
The department has said it wants to reassure students that the early September date is in time for applications to the British UCAS system.
The department says this year's Leaving Certificate exams – for those who wish to sit them – will run in November, subject to public health guidance.
From midday on Monday students will be asked to log in to Gov.ie/LeavingCertificate to confirm that they wish to receive Calculated Grades.
They will receive a text message the following week that they will be asked to respond to.
In a statement this evening, Education Minister Norma Foley said enormous care had been taken at school level to collect the information about students and to make sure that estimated marks are based on reliable evidence.
She said she wanted to assure students that "the people working on this are taking every precaution to ensure fairness so that students can receive the grades that reflect their work".
"The date of 7 September allows us to achieve this", she said.
The minister said students hoping to study in Irish colleges and universities would be able to start their courses "a few weeks later".
Sinn Féin Education Spokesperson Donnchadh Ó Laoghaire said this evening's announcement was "a massive blow to students and to their families".
He said: "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."
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.
Any student who is unhappy with a Calculated Grade in any subject can seek an appeal and also opt to take the written examination in that subject.
However, appeals are limited to how marks arrived at by teachers were processed by the department. The estimated mark and ranking reached by a teacher cannot be appealed.
Applications for appeals will open on 14 September.
The State Examinations Commission says it hopes to be able to run the Leaving Certificate examinations during the month of November.
A small number of students will not be able to receive Calculated Grades in all subjects. They include students where there is no credible source of independent evidence on which to base a Calculated Grade, such as those who were entirely self-taught or taught by a parent.
The Department of Education says the Calculated Grades Executive Office has been processing applications and reviewing cases with schools and decisions will be issued to students in the coming weeks.