THE final cost of the Mahon tribunal will far surpass the €250m estimate released by the government last week.

Experts believe the legal bills for many of the 420 third-party witnesses have yet to be processed and that legal firms will charge the state interest on delayed payment.

The claims follow revelations last week that Department of the Environment projections for third-party legal fees of around €147m will push the overall tribunal cost to about €244m.

However, according to Brief Counsel, a Dublin-based legal firm specialising in procurement and standards, the estimate held by the department is unreliable.

"The figures that have gone out are probably on the conservative side. I would have thought it would have been significantly more," said barrister Hugh Kennedy.

"I would question the methodology by which they came to the €147m – it's not at all clear that they would come to that small a number for that number of third- party witnesses.

"I would say some parties will ask for interest on bills that have not been paid to date."

According to Kennedy, there are few restrictions in tribunals here as opposed to other jurisdictions.

A third-party witness can retain a solicitor who may feel the need to appoint as many as two senior and two junior counsel to act on the client's behalf for up to two weeks of evidence.

This results in a consultancy fee, a fee for administration, fees for senior barristers and for junior counsels who apply the 'two- thirds' principle of being paid that fraction of their senior counterpart's bill.

"They will lash it in and see what they can get away with, with minimum back-up explaining exactly what they are being billed for," said Kennedy.