Around 6,500 Leaving Cert students received a lower grade than they should have, the Education Minister has confirmed.
Norma Foley has detailed the two coding errors have been discovered in the calculated grading process, and acknowledged the issues shouldn't have happened.
Any students who were due a higher grade will have their "proper grade restored".
Some other students received higher grades as a result of the issue, but the Minister has confirmed their grades will not be changed as a result of the error.
Minister Foley has insisted the errors 'will not disadvantage any student'.
For students who will now have their grades upgraded as a result of the error, the CAO will look to see if they're entitled to a higher preference third-level course than they were offered.
It has not yet been confirmed how many students will receive a higher offer.
'One line of code'
Minister Foley said an initial error was discovered last week by a Canadian firm - Polymetrika - which developed the statistical software used to analyse students' data.
One line of code out of 50,000 had a mistake that impacted the way students' grades were calculated, in particular how students' Junior Cycle results were factored in.
She explained: "The system was meant to draw on the subjects of Irish, English and Maths, and combine them with students' two best non-core subjects.
"The coding error instead combined them with students' two weakest non-core subjects."
Further examination led to the discovery of another error - that results in CSPE which should have been discarded in the system were not.
Around 7,200 grades and 6,500 students have been impacted.
Every student impacted by this will be contacted by text message as soon as their result can be confirmed.
Minister Foley has apologised to Leaving Cert students, saying they don't need this "upset and dismay".
She said: "Once the correct grades have been issued, the CAO will establish how many would have been eligible for a higher preference offer in previous rounds of the CAO process.
"Once these people have been identified, the Department will work with the CAO and higher education institutions to see how these students can commence the courses."
A helpline for Leaving Cert students has now been opened on 01-889-2199 in response to the latest development.