The High Court has granted an application by Denis O'Brien to join businessman Declan Ganley to an action against PR firm Red Flag Consulting for alleged conspiracy and defamation.
Mr Ganley had denied he was the client for whom an allegedly defamatory dossier on Mr O'Brien was prepared.
He had opposed the application to join him to the proceedings.
He said there was no evidence he was the client and it was merely Mr O'Brien's belief and the belief of former Fianna Fáil TD Colm Keaveney.
Ms Justice Miriam O'Regan granted the application to join Mr Ganley and allowed Mr O'Brien to amend his claim to enlarge the conspiracy claim beyond what was contained in the dossier.
Lawyers for Mr Ganley said they wanted time to consider whether or not to appeal.
The matter will be back before the court again on 10 April.
