Businessman Denis O'Brien is to appeal a High Court ruling refusing to order a PR company to disclose documents which would reveal the identity of a client who directed them to prepare a dossier about him.

Mr O'Brien began legal action against Red Flag Consulting in 2015, claiming they were involved in a conspiracy to harm his interests and that the material in the dossier is defamatory towards him.

Mr Justice Colm MacEochaidh said the court had not heard the "full story" about the "mysterious appearance" of how the dossier arrived on a memory stick at Mr O'Brien's offices in October of last year.

Lawyers for Mr O'Brien claimed revealing the identity of the client would demonstrate that the client's motivation was to injure Mr O'Brien.

But Mr Justice Colm MacEochaidh ruled the identity of the client was not relevant to the businessman's action against Red Flag.