RTÉ News has learned that Denis O'Brien has acquired a further 5% of the shares in newspaper group Independent News and Media.
This brings his stake in the company to 27%.
The company's share price rose by almost 15% this afternoon in Dublin following a transaction in which 32.7 million shares in the company changed hands at 34.5 cent per share.
This represents a 24% premium to the 27.8 cent the shares were trading for at that time.
Market sources have confirmed Mr O'Brien bought the shares.
A spokesman for Mr O'Brien declined to comment when asked whether the businessman was the investor behind the transaction.
Mr O'Brien, who was already the largest shareholder in the media group, consolidated his control over the company last month when rival Gavin O'Reilly stepped down as chief executive.
An Independent News & Media spokesman said they had not been informed of the identity of the purchaser.
Mr O'Reilly's departure brought to an end his family's 30-year control of Ireland's largest media company and ended years of squabbling between Mr O'Brien and the O'Reillys.