The Court of Appeal is hearing businessman Denis O'Brien's appeal against the High Court's refusal to order a public relations and lobbying firm to reveal the identity of a client.

Mr O'Brien claims a dossier about him was compiled by Red Flag Consulting for a client, for a "hostile" motive, and was part of a campaign of defamation intended to damage him.

He says he is entitled to know whether the client is his "absolute sworn number one enemy in the world".

Mr O'Brien has appealed the High Court's refusal to require Red Flag to disclose documents which would reveal the client's identity.

He also wants the company to disclose documents relating to the alleged publication of the dossier.

He says he needs to know the identity of the client so he can consider whether to sue that person, as well as Red Flag.

Mr O'Brien claimed the dossier, made up of media reports and a number of other documents was on a USB memory stick in an envelope which was put anonymously on his desk at his Dublin offices in October 2015.

Red Flag said the dossier included its material and it was entitled to keep its client's identity confidential.

The firm denies defamation or conspiracy and has raised issues about how Mr O'Brien got the dossier.

Mr O'Brien's Senior Counsel, Michael Cush, told the court this was a "very unusual" case.

He said the question of motive was absolutely central to the claims.

Mr Cush also said it was public knowledge and relevant to motive in this case that Gavin O'Reilly, the Chairman of Red Flag, was the son of Dr Tony O'Reilly, with whom it was very well known that Mr O'Brien had a fractious relations.

Mr Cush said Tony O'Reilly was ousted from INM by Mr O'Brien and Gavin O'Reilly was also ousted from that company.

He said Karl Brophy, Red Flag's Chief Executive, was also a former journalist at INM.

Mr Cush said those previous connections with Mr O'Brien were obviously relevant.

The court was told Mr O'Brien rejects Red Flag's claim that it prepared the dossier in the ordinary course of business.

Mr Cush said it was also quite sinister that the dossier included proposed changes by Red Flag to a speech of then Fianna Fáil TD Colm Keaveney.

He said Mr O'Brien did not accept Red Flag's insistence that the amended speech was not part of its work for the client.

The Court of Appeal has reserved its judgment.