Seventy part-time teachers have received overpayments of up to €12,000 each from the City of Dublin Vocational Education Committee (CDVEC).
The VEC is meeting some of the teachers involved today and tomorrow in an effort to recoup the money - but it is not clear if this will be possible in every case.
About 15 of the teachers are no longer employed by the VEC, while some are thought to be abroad.
It is understood two of the cases involve sums of over €10,000, while about 15 involve overpayments of over €5,000. Most of the overpayments are in the €2,000 to €5,000 range.
The mistake happened because the VEC miscalculated arrears due to the teachers. The teachers were only informed of the error in recent weeks, although it appears the error first came to light in 2004.
The teachers come from some of the best-known VEC schools in Dublin, including Kylemore College, Ballyfermot, Coláiste Dhulaigh in Coolock and VECs in Inchicore, the Liberties, Rathmines and Larkin Community College in the north inner city. Youthreach teachers in Bonnybrook, Sherrard Street and Pleasants Street also received overpayments.
In a letter to the teachers in question, the CEO of Dublin VEC, Jacinta Stewart, writes: "I wish to apologise to you for an error which occurred recently in the calculation and payment of arrears in your salary."
Ms Stewart promises that "all reasonable arrangements for the recoupment of these arrears will be considered".
The CDVEC employs about 4,000 staff, 1,650 of whom are part-time teachers.
In a statement to The Irish Times last night it explained how 140 part- time staff secured new pay conditions under the Protection of Employees Work Act 2001.
"The calculations of arrears, going back to 2001, due to these teachers were complex and detailed. An overpayment was made to approximately 70 teachers.
"The organisation deeply regrets this error. Steps have now been taken to ensure that such errors will not recur."