Derek M Quinlan (born 4 November 1947) is an Irish businessman prominent in the field of real estate investment and development. A former tax inspector at the Irish Revenue Commission, he formed investment syndicates with high-net-worth individuals to acquire investment properties across the world. His principal investment vehicle was Quinlan Private, a private equity firm with offices in Dublin, London and New York.

Quinlan's period of greatest prominence and success coincided with the peak of the global real estate bubble in 2004–2007. In 2009, he resigned from Quinlan Private and moved to Switzerland on the advice of KPMG. His loans have since been transferred to NAMA (National Asset Management Agency) and various assets have been sold, including artwork from his private collection. In September 2014, it was reported that he had reduced his debts by more than €3bn through a series of asset sales over the previous five years.

In 2017 Quinlan returned to the European property market acting as an adviser for several deals, including one of the largest property bids in European history worth an estimated £5 bn.

In November 2022 he declared himself bankrupt in the High Court in London.

Quinlan was born in Dublin in 1947, the son of an Army Officer. He attended Blackrock College and University College Dublin. He began his career as an accountant with Coopers & Lybrand (now PwC) before joining the Irish Revenue Commissioners as a tax inspector. In 1989 he went into private practice, founding Quinlan Private as an asset management firm for high-net-worth individuals. He specialised in investing in tax designated areas in the 1990s in Ireland supporting Irish government policy in relation to property renewal, regeneration and job creation.

The 2008 financial crisis severely impacted Quinlan's liquidity. In 2009, Quinlan resigned from Quinlan Private and moved to Switzerland on the advice of KPMG with the company later rebranding as Avestus Capital.

In April 2011 a receiver was appointed on behalf of the National Assets Management Agency (NAMA) to take charge of a number of properties owned by Quinlan, after he had failed to repay loans to the agency totalling hundreds of millions of Euros. In September 2014, the Sunday Independent reported that he had reduced his debts by more than €3bn, the biggest single repayment of debt by a single borrower owed to the National Asset Management Agency and other banks.

In November 2022 he declared himself bankrupt in the High Court in London.