The High Court has been asked to decide if businessman Denis O'Brien should be allowed to call evidence from a US law expert to support his legal action over statements made in the Dáil about his banking affairs.

In a pre-trial hearing on the admissibility of a 60-page report by a Harvard law professor, lawyers for Mr O'Brien said the case was unique and had never before been determined by the Irish courts.

Senior Counsel Michael Cush said Mr O'Brien should be allowed to submit the report to support his case.

The court was also told Mr O'Brien will also give evidence in the case which is due for hearing later this year.

However, lawyers for the State are opposing the admissibility of such evidence and say it has no relevance to Irish constitutional law.

They said Mr O'Brien's case could turn into "a constitutional convention" if such evidence were allowed into the case.

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 injunction proceedings against RTÉ relating to a programme about his banking with IBRC effectively decided the legal case in whole or in part.

He claims that amounts to interference with the operation of the courts and breaches his rights of privacy and access to the courts as guaranteed under the Constitution and European Convention of Human Rights.

Mr Cush said it would be "appropriate and helpful" to look at authorities from other jurisdictions. He said the issue to be examined was the interplay between the various pillars in a democracy.

There were issues relevant to the case which were shared with other jurisdictions he said, such as the explicit separation of powers which was written into the Constitution.

Mr O'Brien wants to admit a report from US Constitutional Law expert Professor Laurence Tribe as part of his case.

However, counsel for the State Maurice Collins said it was clear from the report there is "simply no US authority that could be seen to bear directly on the issues in these proceedings".

He said if US law was deemed to be relevant then so too could precedent from Australia, New Zealand, South Africa and other countries.

Mr Collins said in practical terms for the running of the trial it could turn into "some form of constitutional convention where the court is hearing submissions about the constitutional law of every common law jurisdiction".

He said the issue in Mr O'Brien's case "is not an issue of American law, it is an issue of Irish law", he said.

Professor Tribe's report was "not a report on US law, its purpose is to advocate a result in these proceedings."

He said it was an attempt to say his views on US law should be taken as evidence on Irish law and that was simply not admissible.

He said Mr O'Brien would be able to refer to US case law in the normal course of the case.

Mr Justice Peter Kelly will give his ruling at a later date.