"Mr. Haughey lived a lifestyle and incurred expenditures vastly beyond the scale of the public service entitlements which were his sole apparent income," the report said.
How Haughey could fund a lifestyle that included an 18th-century mansion, a yacht, private island, stud farm and a lavish wine cellar was the talk of Ireland for years, but few would have questioned the charismatic politician in his prime.
It was only after he retired from public life in the 1990s that details of secret payments, unpaid taxes and offshore bank accounts came fully to light.
Tuesday's report - the first since the tribunal was established - identified $15m in funds available to Haughey between 1979 and 1996, excluding his income from public office and other businesses.
"The vast majority of payments ... were made in forms that were secretive, opaque and frequently involved off-shore vehicles," the report by judge Michael Moriarty said.
It rejected suggestions by Haughey - a former accountant - that he knew nothing about his finances, as well as his argument that personal donations were from "disinterested citizens seeking to assist a politician whose views they supported".
"Apart from the almost invariably secretive nature of payments from senior members of the business community, their very incidence and scale, particularly during difficult economic times nationally ... can only be said to have devalued the quality of a modern democracy," the report said.
Haughey, prime minister three times between 1979 and 1992, dominated Irish politics during a period when the country was saddled with massive debt and huge unemployment, and is often credited with laying the groundwork for Ireland's economic boom.
But his legacy has been tainted by a series of scandals and accusations of corruption.
In 2003, after an epic legal battle, Haughey agreed to pay back taxes and penalties of $6.6m on more than $13m he received in secret payments from businessmen while in office.