Desmond was born in Macroom, County Cork in 1950 and grew up in Marino, Dublin. He was educated at Scoil Mhuire, Marino and Good Counsel College in New Ross. He left school in 1968 to work at Citibank in Dublin.
In 1981, he founded National City Brokers in Dublin. The company competed with already established stockbroking names in Dublin such as Davy's and Goodbodys. Desmond sold the company to Ulster Bank in 1994, which at the time was part of the National Westminster Group, for a reported £39 million. Subsequently, as a result of Natwest's takeoever in 2003 by RBS, NCB was bought out by its management with the support of former Billionaire businessman Sean Quinn who is believed to now control 25% of the company.
Mr Desmond gave evidence relating to the Century Radio module of the Flood Tribunal. He said he had given former Fianna Fáil press secretary PJ Mara a loan of £46,000 between 1986 and 1989, as Mr Mara claimed he had run into financial difficulties. He said he made the payments by cheque.
Mr Desmond also paid £75,456 for the refurbishment of Mr Haughey's yacht, the Celtic Mist, between 1990 and 1991, at a time according to the Tribunal, that Mr Haughey earned between £69,764 and £72,354 a year as Taoiseach. Mr Desmond again claimed this payment was a loan, but the Tribunal disagreed, noting again that Mr Haughey had settled with the Revenue Commissioners in relation to the sum, having failed to pay Capital Gains Tax at the time. The Tribunal also found that the yacht itself amounted to an indirect benefit to Mr Haughey. This was it despite being unable to find the source of the £167,073.90 paid for the yacht.
Desmond also loaned money to Feltrim plc, which was at the time managed by Charles Haughey's son, Conor Haughey. In August 1991, Desmond loaned the company £55,000, made up of £40,000 loaned on 12 August 1991 and £15,000 on 28 August 1991. Conor Haughey told the Tribunal that the company was in danger of being liquidated and he approached Desmond for the money. The loan was never ultimately repaid, and was instead converted into equity.
The Moriarty Tribunal found that Mr Desmond made substantial payments to Taoiseach Charles Haughey. In September 1994 Mr Desmond made a payment of £100,000.00 sterling to Mr Haughey and in October 1996 he made a payment of £25,000 sterling. While Mr Desmond claims these payments to be loans, repayable by Mr Haughey, the Tribunal did not accept this explanation. Ultimately Mr Haughey was forced to settle with the Revenue Commissioners, as it appears he had failed to declare the payments from Mr Desmond.
Desmond purchased London City Airport from Mowlem for £23.5m in 1995. The investment was considered a large risk as London's Docklands was in recession and the neighbouring Canary Wharf development was in receivership. The airport has since become one of the more profitable in the United Kingdom. Desmond sold London City Airport in October 2006 for a reported £750 million to a consortium consisting of insurer AIG, GE Capital and Credit Suisse.
In 1996 Desmond backed a consortium headed by Fran Rooney to purchase Baltimore Technologies for £300,000. The company quickly purchased an e-commerce license from the EU and it experienced rapid growth over the following years. It was briefly a member of the FTSE 100 and had a market capitalization of £4.5 Billion. The company though fell victim to the Dotcom bust and all but collapsed.
Mr Haughey's son Conor told the Tribunal in 1999 that he had learned money to refurbish the yacht had come from the bank account of Freezone Limited. Money from the sale of the Johnston Mooney & O'Brien site had ended up in the same account – a revelation that Conor Haughey said he was "very concerned" about.
BETDAQ is the trading name of Global Betting Exchange, a betting exchange operator based in Ireland. It is the second largest betting exchange operator in the market, with Betfair being the largest. The company was founded in 2000 by Dermot Desmond and started trading in September 2001. Its headquarters are also located in the International Financial Services Centre in Dublin. Desmond sold BETDAQ to Ladbrokes for €30 million in 2013 and is a shareholder at the bookmaker as a result of the deal.
Desmond previously held a stake in Manchester United. He sold out to Malcolm Glazer in 2005.
Desmond was a major shareholder in Irish food company Greencore for a number of years. He sold his shares in 2006.
In August 2008, Desmond was offered the position of chairman of the Irish airline Aer Lingus. Desmond turned down the offer citing prior commitments as a factor which would not allow him the sufficient time necessary to do the job. Former GPA chief operating officer and current Babcock & Brown Air chief executive Colm Barrington was subsequently named chairman of Aer Lingus.