“This time of the year is so hectic,” she said. “It’s just like St. Patrick’s Day on turbocharge.”
Ms. McDonald is a sharp contrast from Mr. Adams. She is the first woman to lead the party’s modern incarnation and did not take part in the sectarian conflict known as the Troubles, which killed roughly 3,500 people from 1968 to 1998. And she is not from Northern Ireland.
In an interview, she said she hoped American support could help Sinn Fein manage two overlapping crises in Northern Ireland: the collapse of its power-sharing government and the looming British exit from the European Union — a process known as Brexit — which could reimpose a border between the Republic and the North.
“Gerry’s shoes cannot be filled, but I have my shoes, Michelle’s got her boots and you’re all going to need your sneakers,” she told a ballroom full of American supporters on Friday. “But we’re in this together.”
American donors have generously backed Sinn Fein since it began fund-raising operations in the United States in 1994, and its leaders are quick to note how important donors, like organized labor and the philanthropist Chuck Feeney, are to the peace process.
Ms. McDonald said she wanted Irish-Americans to play “a very active role” in helping her navigate Brexit. She believes that the turmoil could yield the holy grail of Irish nationalism: the reunification of Ireland.
“The Good Friday Agreement provides for a referendum on Irish unity,” she said, referring to the 1998 peace deal. “My objective is to achieve that referendum, to win that referendum in a context of not triumphalism or certainly not coercion but in a spirit of friendship and consensus and change and modernity and moving forward.”
Larry Downes, who led Sinn Fein’s fund-raising in the United States from 1995 to 2012, said American donations helped pay for Washington lobbying efforts, party buildings in Belfast and London, and years of office supplies. (In recent years, British and Irish laws have limited the political use of foreign funds, he said.)
The Irish Times reported in 2015 that the party had raised $12 million in the United States since 1994, a figure confirmed by Ciaran Quinn, a spokesman. Since that report, Sinn Fein has raised more than $922,000 in the United States, according to disclosure forms filed with the Justice Department.
“In America that’s nothing, but over there it’s huge,” Mr. Downes said. No other party in Ireland has built a similar American fund-raising operation.
Nonpartisan Irish groups in America have raised far larger sums — the Ireland Funds, a philanthropic group, has raised more than $600 million since 1976 — but experts said the success of the Irish republican Sinn Fein reflected longstanding dynamics within elements of the Irish diaspora.
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Ted Smyth, a former Irish diplomat who has researched Irish-American attitudes, said those quirks included, for some people, an appreciation of Ireland’s history as part of the British Empire as well as “a very strong emotional attachment to a sense of being Irish.”
“There has been the tradition since at least 1916 of the Irish in America being more militant than the Irish in Ireland,” Mr. Smyth said. He attributed that to a feeling among earlier generations of immigrants that “they were exiled from Ireland, in a sense, because of British colonialism.”
Those sentiments also benefit the government of Ireland — which has a population roughly half the size of New York City — by allowing it to punch above its weight diplomatically, he said.
Look no further than St. Patrick’s Day. The holiday is widely celebrated in America, even by non-Irish, and includes a series of Washington events for Irish leaders, including a shamrock-themed White House visit and a lunch hosted by the Speaker of the House.
“Without that we’d be New Zealand,” he said.