Gavin Booth and Darragh Mackin of KRW Law were two of four authors of the Report on the Concentration of Media Ownership in Ireland, commissioned by the GUE/NGL group in the European Parliament.

The report set out concerns over the number of legal proceedings brought by Mr O’Brien against journalists and media organisations, as well as “serious shortcomings in the defamation framework” in Ireland.

Following the publication of the report, Mr O’Brien issued a statement which Mr Booth and Mr Mackin believe implied that they were paid by the IRA and their report was not independent.

A letter of complaint reads: “Neither Booth, Mackin nor KRW Law were paid by Sinn Fein or the IRA for the report.”

They say that Mr O’Brien “sought to attack their credibility and independence, and accused them of being paid and influenced by a proscribed organisation” and had put their lives “at risk”.

The pair filed defamation proceedings last week.