O'Brien, who purchased the shares on Tuesday for an undisclosed price, has renewed his call for IN&M to dispose of the loss-making The Independent and for Sir Anthony O'Reilly, the chief executive of IN&M, to resign.
O'Reilly, also a billionaire, and his family control around 27% of the company, which operates in Ireland, Britain, South Africa, Australia, New Zealand and India.
O'Brien used an IN&M shareholder meeting in June to issue a report on the company. He criticised the board of director's overly close links with O'Reilly and recommended The Independent and a portfolio of Australian radio stations be sold.
O'Reilly returned O'Brien's fire, calling the report defamatory and threatening to sue.
The Independent newspaper suffered a considerable loss in sales during October, down 4.51% on September to 240,134 copies. Its six-month average was down 4.61% to 242,685, according to the latest Audit Bureau of Circulation figures.