The press ombudsman Peter Feeney has expressed “deep, deep concern” over an alleged major data breach at Independent News & Media (INM).

The Data Protection Commissioner Helen Dixon has announced she is to conduct an investigation into the alleged breach.

It is alleged to have occurred in 2014 and to have involved emails and other material being searched by third parties for mentions of “persons of interest”.

A number of 19 “persons of interest” identified in a lengthy affidavit from the Director of Corporate Enforcement, Ian Drennan, as being the focus of searches on the leaked INM data were named on Thursday by the Irish Independent.

His affidavit was submitted as supporting a High Court application that an inquiry be sanctioned into possible unlawful activity at the INM group.

The persons named included broadcaster and Sunday Independent journalist Brendan O’Connor, former Independent journalist Sam Smyth who reported extensively on the Moriarty Tribunal, two barristers Jerry Healy SC and Jacqueline O’Brien SC, who previously worked for the Moriarty Tribunal, and a number of former senior executives at INM.

“As Press Ombudsman it is deeply disturbing that communications between journalists and third parties can be accessed and used for purposes other than for which they originated,” Mr Feeney told RTÉ’s Morning Ireland.

“This gets to the core of investigative journalism in the sense that it endangers the confidentiality which a journalist may guarantee someone if he or she is giving them information which may be important to bring into the public domain, it undermines investigative journalism and therefore it is a deep, deep concern.”

Mr Feeney pointed out that a lot of journalism depends on people passing on information to journalist. “The journalist then goes and investigates it and if there’s a story it will be published,” he added.

“If a person has a doubt whether the guarantee of confidentiality means anything then they won’t pass on that information and journalism will suffer.

“Not just journalism will suffer, it’s Irish society will suffer because the Fourth Estate role of journalism, investigating those in authority and those in power, is undermined if confidentiality is threatened.”

He said that it’s probably premature to speculate too much about what was the real explanation for what happened as there are two inquiries taking place.

“We’re speculating without all the information, but it is the case that the management at Independent News and Media would wish to reassure their journalists that they can continue to do their jobs.

“It appears to me - and this is just speculation - that this interrogation of emails was not known by senior editorial managers in Independent papers, they themselves must feel badly let down by what has happened.

“Giving assurances to their journalists is terribly important and I hope it means the reestablishment of the importance of confidentiality as soon as possible in the newspaper.”

The Data Protection Commissioner Helen Dixon said on Tuesday: “We are currently scoping the parameters of that investigation, including seeking additional information from INM this afternoon on foot of the breach notification it filed with the office last week”.

Also among those named on the list was Mandy Scott, former personal assistant to the former INM chief executive, Gavin O’Reilly.

According to another of those named, Karl Brophy, former corporate affairs director with INM, Ms Scott would have had access to his emails, Mr O’Reilly’s emails, and those of the former INM chairman and shareholder, Anthony O’Reilly, father of Gavin.