Trinity College Dublin accidentally undercharged students €1.7 million in fees over the last three years.
An administrative oversight relating to the Susi grant is being blamed for the error.
The college says it will pursue students for €1.7 million in unpaid fees after it accidentally undercharged them.
Over 500 students who applied for the grant but were unsuccessful were mistakenly charged lower fees by Trinity - as if they had been approved.
The college believed it was owed the money from Susi, when in fact individual students owed the money - a mistake that wasn't caught for three years.
80% of students owe €3,000 or less, but other students and some alumni are facing bills of up to €9,000.
In a statement, Trinity says they have put an interest-free payment system in place to help students who cannot pay their fees immediately.
It added that they recognise that this will cause difficulties for some students, but believe its only fair to the vast majority who pay their fees in full and on time.