The State received €2m less than it should have in tolls from the company running the West Link bridge in 2002 and 2003, according the latest report by the Comptroller and Auditor General.
The shortfall was not picked up by the state, the National Roads Authority, or the auditors for NTR - the private company that runs the West Link. The report also reveals that motorists have paid more than anticipated towards the cost of the bridge.
Under an agreement between NTR and the State, the Exchequer is entitled to a share of toll revenue based on volumes of traffic. But the amount due to the State was miscalculated, resulting in a shortfall of almost ¦2m.
The discrepancy arose because of complex compensation payments worth €6.4m made by the state to NTR when VAT was imposed on tolls. NTR accepted that an underpayment occurred, and has repaid the money.
A spokesman for Minister Martin Cullen said the money was paid back with interest and penalties in July.
The actual shortfall was €1.86m, while the interest and penalties came to a further €730,000.
The report also reveals NTR's return on investment has risen from 18% to 24% - mainly due to increased volumes of traffic. This return is two and a half times higher than expected. But the big winner is the State - recouping 52% of all gross toll receipts, between corporation tax, municipal rates and income tax.