Labour TD Brendan Howlin,. who helped blow the whistle on corruption in An Garda Síochána in Donegal, was today ordered to reveal details about his trusted informer.
The five-judge Supreme Court ruled that Mr Howlin must comply with an order made by the chairman of the Morris Tribunal to identify his source.
Mr Howlin had been informed in late 2000 that two senior gardai in the Donegal division were acting improperly.
He brought this information privately to the attention of Justice Minister Michael McDowell but refused to go public with the name of his informer.
Brendan Howlin
In February 2003, the Morris Tribunal ordered Mr Howlin, Fine Gael MEP, Jim Higgins, and telecoms company, eircom, to hand over details, including phone records, documents, memos and letters regarding the informant. Mr Howlin refused, and in the High Court in March 2004 the order was quashed.
Mr Howlin had claimed the information was privileged based on law and also non-disclosure in the public interests.
But the Supreme Court today ruled the sources and their information should be handed over to the tribunal.
After the judgment was handed down Mr Howlin said it would have profound implications for the Oireachtas and the conduct of public affairs.
"It appears now that if I had gone into the Dáil and made allegations on the floor of the Dáil I would have had absolute privilege," the Wexford TD said.
"But because I acted absolutely responsibly, and the Supreme Court judgment says I acted absolutely responsibly by bringing these matters to the attention privately to the Minister for Justice for investigation that privilege did not exist and has not been properly exercised."