Dan Rooney, a former US ambassador to Ireland and co-founder of the Ireland Funds charity, has died aged 84.
He helped to set up the charity in 1976 and since then it has raised more than $550m (£440m) for thousands of Irish organisations around the world.
Irish President Michael D Higgins led tributes, saying his contributions to peace and reconciliation had left a "real and tangible legacy".
He said he was "deeply committed to Ireland and the Irish people".
Mr Higgins added that Mr Rooney - whose grandfather was originally from Newry, County Down - was "always conscious of his Irish roots".
'Thousands have benefited'
Mr Rooney was born in Pittsburgh in 1932, into a family which helped to found the American Football team, the Pittsburgh Steelers.
He later became chairman of the side which has won more Super Bowl titles than any other National Football League (NFL) team.
Between 2009 and 2012, the Irish American served as the United States' ambassador to Ireland.
He was appointed to the role by then US president Barack Obama, who described him as "an unwavering supporter of Irish peace, culture and education".
He co-founded his charity with Irish businessman, Sir Anthony O'Reilly, in 1976, with the aim of supporting and promoting peace efforts and Irish culture.
In 1987, their organisation merged with the American Irish Foundation and today, it is known as the Ireland Funds.
John Fitzpatrick, chairman of the fund, said that he was an "utter gentleman and utterly selfless".
"The work of the Ireland Funds is his legacy and Ireland is a better place because of him.
"Thousands have benefited from his care and concern."
Mr Rooney's grandfather, also called Daniel, emigrated from County Down to the US in the 19th Century and opened a saloon in Pittsburgh.
The Newry man's son, Arthur, founded the team that would become the Pittsburgh Steelers in 1933.
In 2008, Dan Rooney was awarded an honorary CBE for contributions to peace and reconciliation in Northern Ireland.
He was also known as the man behind the NFL's 'Rooney Rule', which requires teams to interview at least one black or ethnic minority candidate for a head coach vacancy.