Independent News & Media has said it was never able to determine who placed its former chief executive Gavin O'Reilly under surveillance in 2011, his final full year in charge, despite what the company called an "extensive investigation".

The country’s largest newspaper group denied it had shelved the investigation after Mr O’Reilly’s resignation in April 2012, despite concerns expressed by the company’s security advisers that his phone may have been hacked.

“This matter was investigated by a reputable third-party organisation and no evidence of hacking was found,” an INM spokesperson said.

“The decision not to proceed further was taken whilst Mr O’Reilly was CEO,” INM added.

The company said it still did not know who was behind the surveillance, which involved up to five operatives at a time.

The Irish Times has learned that INM went as far as consulting a Dublin-based law firm to advise it on how to identify who had ordered the surveillance, which the company described at the time as "corporate espionage".

INM said it was satisfied with its conduct on the matter and indicated the issue was now closed.

“INM takes its responsibilities to its staff very seriously, as evidenced by the extensive investigation conducted by a reputable security firm on this matter,” the company said.

On May 31st, 2011, INM discovered that its headquarters in Citywest business park in Dublin was under surveillance. On June 2nd, 2011, the day before INM’s annual general meeting, Mr O’Reilly was followed into the city centre by a third party, leading to the Garda being alerted.

His pursuers were apprehended outside the Four Seasons Hotel in Ballsbridge and they admitted they had been hired by a party they refused to name to monitor O'Reilly's movements. This kind of surveillance is not illegal in Ireland. NM said that despite the continuing mystery around who had its former chief executive under surveillance it was confident that this could not still be happening today. In May 2013 Anne Harris, the editor of INM title The Sunday Independent, said she believed there was a "strong possibility" that her company email had been hacked. INMsaid it was confident however that its security procedures were strong enough to protect its employees.

“INM has strict protocols in place to ensure the security of its staff and any allegations or concerns are immediately and thoroughly investigated,” INM said.