Senate leaders have announced that Vietnam war hero John McCain will lie in state in the US Capitol Rotunda - an honour given to only a few Americans.
Flags are flying at half mast over the White House and US Capitol following his death at the age of 81.
Former presidents George W. Bush and Barack Obama are reportedly set to speak at the funeral.
His wife, Cindy McCain, says her "heart is broken".
He lost out to Barack Obama in the 2008 presidential election - but his old rival says despite their differences, they shared "a fidelity to something higher".
President Michael D Higgins has extended his sympathy on behalf of the people of Ireland.
President Higgins praised his bi-partisanship on a number of issues while Taoiseach Leo Varadkar described him as a wise and remarkable statesman.