[2][3] His business interests have also extended to aircraft leasing (Aergo Capital), utilities support (Actavo), petroleum (Topaz Energy), football (a minority shareholder of Celtic F.C.

[24] In 2000, Telenor made a bid for control of the company, but O'Brien sold it to BT, reportedly making €250 million from the sale.

[27] In the late 2000s, O'Brien began purchasing shares of Independent News & Media (INM), ultimately spending an estimated €500 million to amass a 29.9% stake in the company.

[28][29][30] O'Brien clashed with the company's board, especially former owner Tony O'Reilly,[31] who stepped down from his position as CEO in 2009 and sold most of his INM shares in 2014.

[28] In 2001, O'Brien founded Digicel, a telecom company that operates in the Caribbean, Central America, and Asia Pacific.

Telstra purchased all six Digicel Pacific markets, including Fiji, Nauru, Papua New Guinea, Samoa, Tonga, and Vanuatu.

[57] In December 2013, O'Brien purchased €300 million in debt owed by Topaz Energy to the Irish Bank Resolution Corporation.

[61] O'Brien has attended the World Economic Forum's annual winter meeting in Davos, Switzerland, alongside other billionaires like Bill Gates and George Soros.

[64] O'Brien considered purchasing Doncaster Rovers in the early 2000's, having owned the club's ground Belle Vue since 1998.

[65][66][67][68][69] Between 2008 and 2016, O'Brien gave the Football Association of Ireland (FAI) as much as €12 million to help pay the salaries of senior officials within the organisation, including Giovanni Trapattoni.

[74] In 1998, O'Brien purchased Planal SA, the holding company for the Quinta do Lago golf resort in Algarve, Portugal.

[76] After 14 years, the Tribunal's final report found, among other things, that Lowry, Ireland's then energy and communications minister, assisted O'Brien in his bid to secure a mobile phone contract for Esat Digifone.

[78][79] That same year, the High Court ruled that the State was not entitled to indemnity and contribution from O'Brien's telecommunications company regarding any loss that might arise from the granting of the country's second mobile phone licence in 1996.

[82][83][84] Sam Smyth, a radio show host that aired on one of O'Brien's networks, claimed he was fired as a result of his reporting on the Moriarty Tribunal.

[85] Today FM responded to the claim, stating that "the decision was made to address a decline in listenership and was part of an initiative to improve programming quality."

[87][88] In February 2013, O'Brien sued the Irish Daily Mail for defamation over his numerous appearances in RTÉ news reports on the relief effort after the Haiti earthquake.

[89] The case was the first time a journalist had attempted to use the honest opinion defence before a jury at the High Court since the Defamation Act 2009 became law.

[95][96] Through the acquisitions of Siteserv and Topaz Energy, O'Brien at one time held hundreds of millions of Euros in debt from the state-owned IBRC.

[99] On 16 June 2015, O'Brien sued the Houses of the Oireachtas Commission, the Government of Ireland and the Attorney General over remarks made by Murphy and Sinn Féin TD Pearse Doherty about his banking affairs in the Dáil Éireann.

[105] Several Irish media outlets ignored the injunction by publishing Murphy's comments or referring to their publication on the Oireachtas website.

[106] In mid-June 2015, Justice Binchy ruled that most of RTÉ's report on Denis O'Brien's financial relationship with the IBRC could be published.

[97] O'Brien defended the injunction in an Irish Times op-ed piece, stating that he had been shocked that somebody took confidential files from a bank, altered them, and then leaked them to the press.

[108] On 10 June 2015, a Commission of investigation was established to inquire into IBRC transactions that lost €10 million or more between 21 January 2009 and 7 February 2013.

[116] In August 1997, O'Brien married Catherine Walsh, who helped Communicorp expand into the Czech Republic and who earlier was the head of marketing for Independent Radio Sales.

[131][132] In 2015, O'Brien established a fully-funded fellowship for Irish students to receive an MBA degree from Boston College.

[135][citation needed] O'Brien donated €2,500 to the campaign of independent candidate Mary Davis for the 2011 Irish presidential election.

[137] Sometime after his purchase of Quinta do Lago in 1998, but before Esat Telecom's sale to BT in 2000, O'Brien sold his home in Dublin and established a primary residence in Portugal.

[138] However, in 2013, the High Court officially ruled that O'Brien's home was Quinta de Lago, Almancil, Portugal, and not Ireland, in the 2000/2001 tax year.

[139] While considering the flotation of Digicel on the New York Stock Exchange, a March 2006 filing to the Companies Registration Office (CRO) listed O'Brien's residential address as Sliema, Malta.