O'Brien held the biggest number of shares in Independent News & Media (INM), which consists of dailies the Irish Independent and Sunday Independent, as well as the Belfast Telegraph in addition to smaller regional publications.
A Belgium group Mediahuis offered €145.6m to buy out INM, resulting in a €43.5m payout for O'Brien and €21.8m for the group's second-biggest investor and Celtic football club shareholder Dermot Desmond.
INM's chairman Murdoch MacLennan said in a statement that the deal represented an excellent outcome for both parties.
"The offer from Mediahuis represents a compelling opportunity for shareholders to realise cash for their shareholding in INM, at a price which fairly reflects the company's performance and standalone prospects. The INM Board believes that this offer from Mediahuis, if approved, will herald an exciting new chapter for our employees, readership and customers."
INM accounts for more than 50% of the daily newspaper market in Ireland and more than 65% of the Sunday market.
Belgium's Mediahuis was founded in 2013 and owns a strong portfolio of news media and digital brands, currently employing more than 3,200 people and reporting a turnover of €819m in 2018.