Sinn Féin's transformation from a party of political pariahs to guests at black-tie fundraisers in the United States began in a function room of the Sheraton Hotel in Manhattan one evening in April 1992.

There the presidential candidate Bill Clinton told a forum on Irish-American issues that he would grant a visa to Gerry Adams, who was at that point banned from the US. More significantly, he also said he would seek a new "governing rationale" for US engagement in Northern Ireland.

In other words he would look beyond the “special relationship” with the United Kingdom that had ensured Washington marched in lockstep with London on Northern Ireland policy, which included ostracising Adams and his party.

As president, Bill Clinton kept his promise, and in January 1994 he allowed Adams to visit New York for three days, over the strenuous objections of the FBI, the CIA and the State Department, which viewed it as a reward for terrorism.

The visa decision sundered the special relationship. "The worst rift since Suez," said a Daily Telegraph headline. But the message to militant republicans was that diplomacy could bring bigger international gains in the struggle against the Brits than the gun ever could.

The US president did not act in a vacuum. He had cultivated Irish politicians since his inauguration and opened the White House to St Patrick’s Day banquets. He knew that negotiations were going on to end the conflict and that, to succeed, Sinn Féin and the loyalist parties must be accommodated.

He got the nod on the visa from important participants he trusted: Albert Reynolds, John Hume and his ambassador in Dublin, Jean Kennedy Smith.

Further visas were issued for longer stays, and in March the following year Clinton gave authority for Adams to make a fundraising tour. Again British officials were apoplectic. Prime Minister John Major refused for a week to take a call from the White House.

The president was swayed by his powerful congressional ally Senator Edward Kennedy, who argued that "if we expect Sinn Féin to act like a legitimate political party, we must treat it like one". Kennedy also pointed out that Sinn Féin was allowed to raise funds in the UK.

Literary and entertainment figures contributed to a growing positive image for Adams. The New York Times had a profile by Edna O'Brien comparing him to Michael Collins. In Hollywood he was hosted as a celebrity by Fionnula Flanagan and other stars.

This political and cultural endorsement had a profound effect on mainstream Irish-Americans who had despaired for years of being able to help in a significant way. They were reluctant to be associated with IRA violence by supporting Noraid and frustrated that the only other options were to picket the British consulate or donate to the Ireland Funds.

Soon it became respectable for them to support Sinn Féin as a way of taking the gun out of Irish politics, a phrase Adams used often. Sinn Féin was also assisted in becoming mainstream by the ad-hoc intervention of a group of Irish Americans previously not associated with Irish republican politics.

Organised by the New York publisher Niall O'Dowd, and including among its ranks the philanthropist Chuck Feeney, they had reached out to Sinn Féin and loyalist paramilitaries when no one else would talk to them.

Feeney had decided to do something to help end the violence after seeing the horrific aftermath of the Enniskillen bombing on television in November 1987, and he became enthusiastic about the idea of bringing paramilitaries in from the cold.

The group made a number of visits to Ireland. They travelled to Belfast in August 1994 to be told by Adams that an IRA ceasefire could be imminent. For this to succeed it was important that rank-and-file militants saw the benefits of the democratic process.

This was a factor in Feeney’s decision to provide $20,000 (€18,000) a month for 36 months, from his personal funds, to open a Sinn Féin political office in Washington. “It was the right thing to do,” he would say later. “It proved you can bring people round to your thinking.” Feeney also gave $200,000 (€180,000) to Gary McMichael, a political representative of the Ulster Defence Association. Feeney’s initiative was applauded by the White House. Nancy Soderberg, in Clinton’s national security council, said it fitted their policy that engagement and interaction would lead to moderation

President Clinton visited Belfast at the end of 1995 and shook Gerry Adams’s hand, an act unthinkable when he made his election promises in the Sheraton just three years earlier, and a final endorsement of the Sinn Féin leader’s peace credentials in the eyes of Americans.

Conor O'Clery is a former Washington correspondent of The Irish Times