The revelations came to light after reporting restrictions were lifted yesterday in the High Court. The court heard Mr O’Brien had anonymously been given a memory stick containing an “extraordinary” dossier of documents mostly unfavourable to his interests.

Mr O’Brien alleges a forensic examination of the dossier by a firm, Epsion, indicates Dublin-based public relations firm Red Flag Consulting Ltd, whose executives include Gavin O’Reilly, former CEO of Independent News & Media and former senior executive with INM, Karl Brophy, are linked to the alleged conspiracy.

He believes a client of Red Flag Consulting is behind the alleged conspiracy and wants to establish the identity of any such client.

Mr O’Brien’s barrister said the core allegation is the defendants have been involved in authoring documents as part of the conspiracy. There were a number of “unknowns” concerning who commissioned the defendants to put together the material.

Mr O’Brien believes the dossier was also designed to be given to people who can avail of parliamentary privilege, the court heard.

Mr O’Brien is claiming damages for alleged defamation and conspiracy and his lawyers obtained an interim High Court order preventing Red Flag Consulting interfering with or removing computer material and other IT items from its offices. It was also ordered that nothing could be reported on until the matter came back before the court yesterday so as to allow the proceedings be served.

Applying for the order on Tuesday on an ex-parte (without the knowledge of the other side) basis, Michael Cush SC, for Mr O’Brien, said the defamatory material was in a dossier including eight Microsoft Office documents on a memory stick provided anonymously to Mr O’Brien following an investigation which he ordered due to his belief of a concerted campaign against him.

The material was clearly defamatory and dealt with matters including Mr O’Brien’s involvement with the Moriarty Tribunal, his tax status as a resident in Portugal, the recent initial public offering of shares in Mr O’Brien’s Digicel company and other allegations, counsel said.

A court order permitting access to the company’s IT system would enable his side to identify contributors and receivers to the material in question, he said. While Red Flag involved “people with whom my client had previous dealings”, there was clearly a client involved here as it is a PR firm, counsel said.

However, High Court president Judge Nicholas Kearns said he was not inclined to grant the “quite draconian” order allowing lawyers and investigators enter the Red Flag offices to take control of the computers. Such an order might seem unduly heavy-handed and may be perceived as interfering with the rights of the defendants, he said.

Mr O’Brien’s action is against the Red Flag firm and five persons involved with it. They are Gavin O’Reilly and Karl Brophy; Red Flag chairman Seamus Conboy; account manager Brid Murphy; and account executive Kevin Hiney.

The High Court continued the interim orders for preservation of documents contained in the dossier.

These include PDF documents entitled “Who is Denis O’Brien?” and “The Moriarty Tribunal Explainer”, along with many Irish and international media articles about the businessman which he contends are mostly unfavourable to him.

The international media articles include stories from the Washington Examiner, Boston Haitian Reporter, Bloomberg, Antigua Observer, Reuters and The Wall Street Journal. The dossier also contains a Dáil transcript relating to the Siteserv transaction.

During Tuesday’s proceedings, Mr Cush said none of the articles in the “extraordinary” dossier are “complimentary” to Mr O’Brien and it was clearly put together to brief against his interests.

Mr Cush also said Karl Brophy was a former journalist with the Irish Mirror who wrote an article in 1998 which ultimately led to Mr O’Brien getting €750,000 damages. Mr Brophy later joined INM and blamed Mr O’Brien for his departure from that company, he said.

There was “a very public prior history” between Gavin O’Reilly and the plaintiff, Mr Cush added. Mr O’Reilly’s departure from INM was widely said to coincide with Mr O’Brien taking a very significant shareholding in that firm.

All the defendants except Mr O’Reilly were represented by Michael Collins SC who said he needed time to prepare. He had just come into the case and he anticipated he would get instructions to fully defend it. He also expected to be representing Mr O’Reilly, who is currently in the US.

Mr Collins said they might need to consult with gardaí and take proper instructions before deciding on the proper response.

Judge Colm MacEochaidh returned the matter for mention tomorrow.