Politicians from across the island of Ireland are flying to Boston to pay their last respects at the funeral of Senator Edward Kennedy.
They include Deputy First Minister Martin McGuinness, Sinn Fein leader Gerry Adams, Mark Durkan and John Hume of the SDLP and Taoiseach Brian Cowen.
A book of condolence for Senator Kennedy's family has opened in Belfast at the US Consulate at Danesfort House.
The public can sign it between 1400 and 1600 BST on Friday.
The consulate has also provided an email address to forward condolences to the late senator's family: firstname.lastname@example.org.
In Dublin, a steady stream of people signed a book at the US Embassy on Thursday.
Senator Kennedy will be buried at Arlington National Cemetery near Washington.
The cemetery is also the resting place of his brothers, former US President John F Kennedy and Senator Robert Kennedy.
The Irish Ambassador Daniel Rooney opened the tributes for the book of condolence in Dublin.
Mr Rooney said the senator played a crucial role in the NI peace process.
"Senator Kennedy was not only a great American statesman, but also a great friend of mine and all the island of Ireland and its people," he said.
"He was instrumental in advancing the historic ceasefire in 1994, and he contributed to the talks that brought peace.
"In the Senate, he introduced resolutions condemning all violence in Northern Ireland and expressing support for the Good Friday Agreement of 1998 as the blueprint for lasting peace."
Northern Ireland's political leaders also paid tribute to the Democratic senator from Massachusetts, who died aged 77 on Wednesday.
A US flag flies at half mast in tribute to Senator Kennedy at the American Embassy in Dublin
First Minister Peter Robinson said he was "a politician of world renown whose influence was felt across the world".
Deputy First Minister Martin McGuinness said his passing would be felt by many across the world, "and in particular the Irish diaspora in America".
"Senator Kennedy played a central role in American domestic political life and contributed significantly to the development of the peace process here," he said.
Mr Kennedy was nominated for a knighthood earlier this year for "services to the British-American relationship and to Northern Ireland".
Senator Kennedy was diagnosed with a malignant brain tumour in May last year.
He became a member of the Senate in 1962 to replace his brother when he resigned to become president, and was re-elected seven times.