Education Minister Norma Foley said an error in the calculated grading system will "not disadvantage any student" after she revealed 6,500 students are impacted.
Ms Foley confirmed there were two errors in the calculated grades system.
"These are errors that should not have occurred, however, this will not disadvantage any student," she said.
The fault was identified by a contractor, paid €160,000 to oversee the function of predicted grades, on Tuesday of last week.
The contractor informed the Department of Education immediately but the public was only made aware of the mistake today after a debate in the Dáil.
"Last Wednesday the secretary-general told me a mistake had been spotted by the Canadian company Polymetrica International," Ms Foley said.
"At that point, here is what we know, one line (of code) out of 50,000 lines had a mistake in it, that would impact the results of some students. It was important to find out as much as possible before making announcements. Staff then began a detailed analysis of 50,000 lines of code affecting results.
"The system was meant to draw on the core subjects of Irish, English and Maths and their two strongest non-core subjects, instead, it combined the weakest non-core subjects.
"In the course of the review by Department, staff found a further error in the code, where the Civic Social Public Education module was meant to be discarded and wasn't, as a result approximately 6,500 students received at least one result one grade lower than should've been, that is being rectified.
"At that point, I called in international experts to do an independent review of the code. Those three strands of work will be complete in the coming days and I will make a statement in the Dáil when it is completed.
"In addition to students who received a lower grade, some received higher. They will not be affected in any way, their grades stand.
"Those marked down will have proper grades restored. When all checks are completed, results will be given to students affected.
"Every student will be informed by text if they have been affected or not. If affected they will be redirected to porta with a new statement of provisional results.
"This shouldn't have happened and will be distressing for students who thought this process was complete. On behalf of the department, I want to apologize sincerely for the situation and the upset it will cause.
"Once correct grades are issued, the CAO will determine who should have been offered a place and the department will work to see these students facilitated to a course they may have been offered in an earlier round.
"Students have had an exceptionally difficult year and coped unbelievably well in Covid-19 challenges. They don't need this, I do appreciate that.
"We will do everything to support students who may wish to change their course and will be supported to do so."
Minister Foley says the Taoiseach's department were made aware of the mistake last Wednesday shortly after the Department of Education uncovered the error and the Taoiseach "was as surprised as I was".
Micheál Martin earlier told the Dail: "Our objective has to be the students and how they receive this information."
Opposition TDs and education spokespeople were given a separate teleconference briefing on the issue, in which they were told Minister Foley was made aware of the issue "last week," and the meeting itself was "very brief" as many of the attendants had technical difficulties.
The government says the announcement was planned on Thursday, despite speculation that the error had only been revealed when the Taoiseach was challenged in the Dáil.