Bill Flynn was credited with playing a role in getting the US visa for Mr Adams in 1994, in the face of stiff opposition from the British government.
He was chairman of the foreign policy think-tank that issued the invitation to Mr Adams to come to New York's Waldorf-Astoria hotel for a conference on the peace process.
It has now emerged that Mr Flynn was the person who paid for Mr Adams's "surgical procedure" for a non-cancerous prostate problem in New York last summer.
He grew up in Queens, New York, but his father came from Loughinisland, in Co Down, and his mother from outside Castlebar, in Co Mayo. He had a hugely successful business career, becoming the chairman of the Mutual of America insurance company.
But he was awarded an honorary Commander of the British Empire (CBE) by Queen Elizabeth in 2009 in recognition of his exceptional contribution to the peace process. He played a key role as chairman of the National Committee on American Foreign Policy and by regularly flying to Ireland to meet key figures.
Mr Adams confirmed over the weekend that Mr Flynn had paid the bill for the procedure, which has been estimated at $30,000.
"I didn't pay for it. A very, very good friend of mine, a man called Bill Flynn, very kindly paid for it. I haven't told anybody that before," he told RTE's Marian Finucane show.
It is not the first time Mr Flynn intervened personally to help a member of Sinn Fein.
In 2011, he appeared on a video screen to dismiss the claim by Environment Minister Phil Hogan that corporate investment in Ireland would dwindle if Sinn Fein's Martin McGuinness was elected president.
Mr Adams had been criticised by politicians on both sides of the Border, with an SDLP assembly member, as well as Fine Gael and Fianna Fail politicians, saying his actions were at odds with Sinn Fein policy.
The details of his trip were included in official documents filed with the US government by Friends of Sinn Fein -- the party's US support group which paid for Mr Adams flights.
But Mr Adams has denied that he was a "hypocrite" for opting for private US treatment while his party opposed the privatisation of health services. He also said the procedure was not available in Northern Ireland.
"I have had a long-standing and very, very painful complaint. And I go back and forth to the States once or twice a year. One of my friends introduced me to his doctor who was dealing with these issues and he recommended that I should have a process of a particular kind, in fact, he said it was long overdue," he said.
Mr Flynn could not be contacted for comment yesterday.