O'Brien has sued his first cousin in California. That cousin, Donald MacAllister, is the son of O'Brien's paternal Aunt Maureen, who was killed in a collision with a car driven by a 36-year-old Michael Smurfit in Dublin's Foxrock suburb in 1972. The case centred on private e-mails MacAllister sent to American, Irish, Jamaican and Burmese politicians and media (among them Micheál Martin, Vincent Browne and Aung San Suu Kyi). Statements such as O'Brien being the largest contributor to the Clinton Foundation and O'Brien telling MacAllister that Smurfit had financed O'Brien's first radio station were not denied by O'Brien.
O'Brien studied politics, history and logic at University College Dublin, graduating in 1977. After winning a scholarship from Boston College while attending UCD, he completed an MBA in corporate Finance there in 1982.
O'Brien formed Communicorp in 1989. It launched its radio operations in Ireland that same year and entered the Czech Republic in 1992. It later added stations in Bulgaria, Estonia, Finland, Hungary and Ukraine. Based in Dublin, the company now owns 42 radio stations in eight European countries, including Ireland's Newstalk, Today FM, Dublin's 98FM, SPIN 1038, TXFM and SPIN South West. O'Brien owns a majority of Ireland's radio stations (all stations apart from those run by state broadcaster RTÉ), save some regional and local stations.
O'Brien set up and chaired the Esat Digifone consortium which acquired a mobile phone licence in the 1990s. Esat Digifone's bid defeated five other applicants, some of whom included major international operators, in controversial circumstances that became the subject of investigation by the Moriarty Tribunal. The Esat Digifone consortium was 40% owned by O'Brien's interests, 40% owned by Telenor, the Norwegian state telecoms operator, with the balance being owned by International Investment and Underwriting (IIU), an investment vehicle owned by Dermot Desmond.
On 7 November 1997, Esat Telecom Group plc held an initial public offering and was listed on the Irish Stock Exchange, London Stock Exchange, and NASDAQ Stock Markets. There were differences between O'Brien and Telenor, however, ESAT Digifone boomed. Telenor made a bid for control of the company, but O'Brien chose to sell to BT in 2000. Esat became a wholly owned subsidiary of BT and was delisted from the stock market. O'Brien personally netted €317 million from the sale. The Moriarty Tribunal found almost beyond doubt that O'Brien's was awarded this contract due to payments he made to Michael Lowry, the then communications minister, who unduly influenced the bidding process.
In 1999, O'Brien co-founded aircraft leasing company Aergo Capital, registered in Dublin, with offices in Johannesburg, Nairobi, Santiago and Singapore. It owns and operates a fleet of 103 commercial aircraft. The company is valued at US$250 million and has debts in the region of US$166 million. O'Brien controls about 83 per cent of Aergo and, along with his Father, sits on the company's six-member board. Aergo's clients include Alitalia, Jet Airways, South African Airways, KLM and DHL. In July 2008, Aergo acquired the Safair aircraft-leasing division of Imperial Holdings for €110 million, taking over the division's 33 planes and leasing a further nine aircraft from the parent company. In November 2010, Aergo sold Safair, netting $35 million. Accounts filed in late-2011 showed pre-tax profits increased to $29.4 million over the previous 12 months from $2.68 million in 2009, with the number of employees decreasing from 166 to 15 over the same period.
O'Brien took up residency in Portugal some time before Esat Telecom's sale to BT in 2000. He netted more than €300 million from that sale but paid no capital gains tax due to a then-existing provision in the Irish-Portuguese tax treaty. While considering the flotation of Digicel on the New York Stock Exchange in 2006, it emerged in public that O'Brien had taken up residence in Malta. O'Brien's move to Malta was revealed in a March 2006 filing to the Companies Registration Office (CRO), which listed O'Brien's residential address in Malta. Malta charges no tax on worldwide assets or income brought in by permanent residents. Residence, for tax purposes, means renting or buying a property with a minimum value and visiting Malta at least once within one year of becoming a resident.
O'Brien supported the 2003 Special Olympics World Summer Games, holding the title "Chairman of Council of Patrons." He is also a Director on the U.S. Board of Concern Worldwide.
UCD awarded O'Brien an honorary doctorate in 2006.
O'Brien's Communicorp was the highest bidder for Emap's Irish operations when that company decided to sell its radio stations, buying FM104, Highland Radio and Today FM on 14 July 2007. In October 2007, the Broadcasting Commission of Ireland (BCI) approved Communicorp's proposed takeover of Today FM and Highland Radio, but not FM104. The deal was completed by January 2008. Due to a Competition Authority decision, Communicorp was required to sell-on FM104, which it did (to UTV Media) immediately upon its acquisition. O'Brien offloaded Highland Radio in mid-2008.
O'Brien and the Clintons were prominent Investors of time and money in Haiti following the 2010 earthquake. U.S. government agency USAID, overseen by the State Department and headed at the time by Hillary Clinton, donated millions of U.S. tax dollars to O'Brien's Digicel company. USAID's "Food for Peace" scheme transferred money over Digicel's mobile network, with citizens of the stricken country being given mobile phones and free Digicel accounts, which led Digicel to "significantly expand" its customer numbers. Bill Clinton also oversaw the building and then opened a luxury Marriott Hotel in Haiti; this hotel was owned by Digicel, which made a $45 million contribution (compared to a $26.5 million contribution from the World Bank-affiliated International Finance Corporation).
In 2011, Bill Clinton flew to Ireland on O'Brien's private jet to attend the Global Irish Economic Forum.
According to The Irish Times, Clinton's 2013 speaking engagement at Dublin's Conrad Hotel was "facilitated largely by his friend" Denis O'Brien. When opening his speech, Clinton personally thanked O'Brien "for the invitation." According to U.S. accounts, Clinton's average speaking fee for the previous year a minimum of close to $200,000 (though he had received multiples of that fee at some corporate functions).
On Saturday 19 July 2014, group Editor of INM Stephen Rae ordered the presses to be stopped to amend a column written by Sunday Independent Editor Anne Harris which featured references to O'Brien. Copies of the original article did however appear, allowing comparisons between the two. Harris originally wrote: "Denis O'Brien is the major shareholder in INM. In theory, with 29% of the shares, he does not control it. In practice, he does." Rae had the last sentence deleted. Harris also wrote: "The question is whether he understands newspapers. In order to confront the truths in our society, we must have a free press. With the restrictive charter for journalists proposed last year, and some garrotive (sic) new structures, Denis O'Brien does not make this easy." This was changed to: "The question is whether he understands newspapers. In order to confront the truths in our society, we must have a free press. If the restrictive charter for journalists proposed last year, along with some other structural changes, are anything to go by, it might be instructive for him to Listen to journalists, troublesome and all as they are."
On 16 June 2015 Counsel for Mr O'Brien informed the High Court that he was suing the Houses of the Oireachtas Commission, Ireland and the Attorney General over remarks made by Independent TD Catherine Murphy and Sinn Féin TD Pearse Doherty about his banking affairs (following his above lawsuit against RTÉ) in an alleged breach of parliamentary privilege, violating his constitutional rights and his rights under the European Convention on Human Rights. The Oireachtas Committee on Procedures and Privileges had previously rejected O'Brien's claims that Deputy Murphy's allegations were a breach of parliamentary privilege.
In September 2016, during the 2016 US Presidential election, Republican candidate Donald Trump's campaign produced a lengthy e-mail to the media criticising Hillary Clinton's relationship with O'Brien. O'Brien refused to make any comments on Trump's statements.
On March 24, 2018 the Office of the Director of Corporate Enforcement (ODCE) in Ireland said it would apply to the High Court of Ireland to appoint inspectors to Independent News and Media to investigate an alleged data breach. According to an affidavit filed by the ODCE, invoices associated with the data interrogation were discharged by Blaydon Limited, a company owned by Denis O'Brien, INM's largest shareholder.
Emmet O’Neill, a nephew of O’Brien and formerly chief executive of Smiles Dental, stepped down as managing Director of Topaz following completion of the transaction.