The High Court has refused Denis O'Brien permission to call evidence from a US expert on constitutional law in his legal action over remarks made in the Dáil about his banking affairs.
The application was made in advance of Mr O'Brien's High Court action against the Dáil, the Committee on Procedures and Privileges and the State arising from speeches and remarks made in the Dáil under privilege.
Mr O'Brien alleges that permitting those remarks to be made after he had initiated separate injunction proceedings against RTÉ relating to a programme about his banking affairs with IBRC effectively decided that case against RTÉ in whole or in part.
He also claims it amounted to interference with the operation of the courts and breached his rights to privacy and access to the courts.
The case is due to be heard at the end of this month.
Last week his lawyers brought an application to be allowed call evidence from Professor Laurence Tribe of Harvard Law School who was involved in writing the constitutions of South Africa and the Czech Republic.
Lawyers for the State had argued that such evidence had no relevance to his case which was based on Irish law.
Today the President of the High Court, Mr Justice Peter Kelly, said Mr O'Brien's case is "rooted exclusively in his alleged entitlements under Irish law".
He said no issue of American constitutional law or foreign law arose and Professor Tribe's evidence was inadmissible.
Even if admissible, that evidence was not "reasonably required", within the meaning of new conduct of trial rules, for the court to determine the case, he said. The evidence was "not required at all, much less reasonably required".
While Mr O'Brien's side argued this was a "unique and important" case raising issues of Irish law not previously decided, all cases are unique to their own facts and important to the litigants, he said.
The case raised "interesting and important" questions and the court may well have to decide issues not adjudicated on before but that was no justification for admitting this evidence which, on principle, was inadmissible.
It was open to Mr O'Brien to cite US decisions in his case, an option frequently followed in other cases, he noted.
He also awarded costs of the application against Mr O'Brien.