The secrets of the long-running planning tribunal are set to die with it rather than being pored over in the National Archives by future generations of historians.

This follows an internal ruling that confidential documents be returned to their owners or destroyed before the inquiry is wound up.

Chairman Justice Alan Mahon has ruled all documents other than specified "relevant material" be returned to the person who provided them or their lawyers.

"In the event that the party who provided the documentation does not require the documentation to be returned, the tribunal undertakes to have the documents destroyed," tribunal solicitor Susan Gilvarry states in a letter sent to witnesses.

Relevant material as determined by the tribunal will be deposited with the Minister for the Environment for transmission to the National Archives. These records will be locked away for 30 years before being made publicly available.

‘Relevant material’

The €158 million inquiry, which was set up in 1997 and filed its last report in 2012, is in the process of winding up its affairs. Aside from dealing with legal bills, its main work now is deciding what to do with the 1.5 million documents amassed over its lifetime.

Ms Gilvarry told The Irish Times the tribunal was in the process of complying with the legislation on winding up tribunals. The chairman had determined the "relevant material" to be sent for archiving comprised briefing material provided to witnesses and all documents created "for and on behalf of" the inquiry.

She said other documents would not be sent for archiving and would be returned to their owners or disposed of.

Lawyers for ex-council official George Redmond, who has had findings of corruption against him by the tribunal quashed, have accused the inquiry of "an unnecessarily narrow view" of the documents to be retained and archived.

Last month they threatened to seek a judicial review of the decision if the tribunal went ahead with destroying material relevant to Mr Redmond.