Minister for Transport Paschal Donohoe has offered his support to 10 Oireachtas members listed by businessman Denis O'Brien in his legal proceedings against the Dáil's Committee on Procedures and Privileges.

Speaking in Dublin on Tuesday, Mr Donohoe said Dáil privilege was a cornerstone of Irish democracy.

“Privileges that are available to members of the Oireachtas are an absolutely essential cornerstone to how democracy operates,” he said.

The Irish Times reported on Monday that Mr O'Brien has initiated proceedings against the CPP for its findings that Independent TD Catherine Murphy did not abuse privilege when making allegations about him in the chamber.

The businessman has named each of the 10 members on the committee, including the Ceann Comhairle, Seán Barrett, in the submission to the High Court.

The CPP members are Mr Barrett (Fine Gael), Joe Carey (Fine Gael), John Halligan (Independent), Martin Heydon (Fine Gael), Paul Kehoe (Fine Gael), John Lyons (Labour), Dinny McGinley (Fine Gael), Seán Ó Fearghaíl (Fianna Fáil), Aengus Ó Snodaigh (Sinn Féin) and Emmet Stagg (Labour).

Fianna Fáil leader Micheál Martin said on Monday that the legal proceedings brought by Mr O'Brien against the committee represent a threat to democracy.

He said the “proceedings represent a threat to a fundamental right enshrined in our Constitution, which is freedom of speech in parliament”.

Sinn Féin leader Gerry Adams said it was a disturbing development, noting "none of this is healthy for democracy. It does not serve the interests of citizens. TDs must be free to question the actions of the powerful and the privileged without fear or favour."

Renua leader Lucinda Crieighton said: “I would say very firmly that Renua Ireland defends the principle of freedom of speech in our parliament. We stand behind the Committee on Procedures and Privileges.”

After taking legal advice from the Oireachtas legal counsel, the CPP rejected assertions by Mr O'Brien that Ms Murphy had abused Dáil privilege.

It said it could not determine if her allegations were true or false until the Commission of Investigation into certain Irish Bank Resolution Corporation transactions was complete.

Ms Murphy had made a number of claims in the Dáil about the sale by IBRC to Siteserv, a company owned by Mr O'Brien.

Mr O'Brien's lawyers, William Fry, accused the Houses of the Oireachtas of having no "effective parliamentary procedures to prevent a member abusing parliamentary privilege".