The Supreme Court has ruled that businessman Denis O'Brien must pay most of the legal costs of his failed case over statements made by two TDs in the Dáil about his banking affairs.

The cost to Mr O'Brien is estimated at €1m.

The seven-judge Supreme Court upheld a decision requiring Mr O'Brien to pay the costs of the High Court hearing of his case, brought against the Dáil Committee on Procedure and Privileges, the Dáil and the State.

However, it directed both sides to pay their own costs of Mr O'Brien's unsuccessful appeal to the Supreme Court against the High Court decision.

The Chief Justice, Mr Justice Frank Clarke, said Mr O'Brien had argued he was entitled to costs because he had won on some of the issues raised.

The court considered there was no merit in that argument as points could be totted up in such a way.

However, the fact the Supreme Court had, to an extent, dismissed the appeal on the basis of somewhat different reasoning from that of the High Court is relevant, he said.

That, along with the importance and novelty of the underlying issues and their resolution, would warrant "some departure" from the normal rule on costs.

The Chief Justice said the court considered the justice of the case would be met by leaving the High Court order on costs in place but making no order on the costs of the appeal, with the effect both sides pay their own costs of the appeal.

Mr O'Brien's case arose from statements separately made in the Dáil by Social Democrats TD Catherine Murphy and Sinn Féin TD Pearse Doherty in May and June 2015.

He complained those made pointless an injunction he had obtained against RTÉ some weeks earlier restraining it broadcasting details of his banking relationship with State-owned Irish Bank Resolution Corporation.

He went to the High Court after the Committee on Procedure and Privileges (CPP) rejected his complaints about the TDs' actions.

The High Court dismissed his case saying it was impermissible under the constitutional separation of powers and would have a "chilling" effect on the freedom of parliamentary speech.

The core issue before the Supreme Court was whether the courts can intervene when an Oireachtas committee allegedly fails to obey its own rules.

Last March, a seven-judge court ruled any interference by the courts in how the CPP had dealt with Mr O'Brien's complaint over the TDs' actions is impermissible under the Constitution.

Such interference would amount to an "indirect and collateral" interference by the courts with statements made in the Oireachtas which are protected by article 15, which confers privilege on parliamentary speech, it said.