(CNN) Former US President Bill Clinton has led tributes to Martin McGuinness, describing the former Irish Republican Army commander and Deputy First Minister of Northern Ireland who died on Monday as "courageous."

McGuinness became Sinn Fein's chief negotiator during the Northern Ireland peace process , working with Clinton on the 1998 Good Friday Agreement. He died on Monday night after a short illness, according to a statement released by the Sinn Fein party. He was 66.

"When he decided to fight for peace, Martin was calm, courageous, and direct," Clinton and former US Secretary of State, Hillary, said in a statement.

"And when he gave his word, that was as good as gold. As Sinn Fein's chief negotiator, his integrity and willingness to engage in principled compromise were invaluable in reaching the Good Friday Agreement.

Both Bill and Hillary Clinton worked with McGuinness during their careers.

"My lasting memory of him will be the pride he took in his efforts to improve disadvantaged schools in Unionist and Protestant communities. He believed in a shared future, and refused to live in the past, a lesson all of us who remain should learn and live by. May he rest in peace."

McGuinness died less than three months after resigning as Deputy First Minister, sparking an election and threatening Northern Ireland's fragile power-sharing arrangement.

He retired from politics on January 19, 2017, saying his health had been deteriorating.

In a statement, Sinn Fein described him as a man of "great determination, dignity and humility."

"He was a passionate republican who worked tirelessly for peace and reconciliation and for the re-unification of his country. But above all he loved his family and the people of Derry and he was immensely proud of both," Sinn Fein President Gerry Adams said in a statement.

McGuinness and Adams were hugely influential the Northern Ireland peace process.

Under their tribute, Sinn Fein wrote in Irish, "I measc laochra na nGael go raibh a anam dílis," which translates as "May he rest in peace in the presence of Irish heroes."

He is survived by his wife Bernie and four children.

'I am an Irish Republican'

McGuinness grew up in Derry, the epicenter of "The Troubles," Northern Ireland's decades' long sectarian conflict.

JUST WATCHED Historic moment McGuinness met Queen Elizabeth II Replay More Videos ... MUST WATCH Historic moment McGuinness met Queen Elizabeth II 00:44

About 3,500 people were killed during the Troubles, a conflict between Catholic Irish nationalists, who wanted to join the Republic of Ireland, and Protestants who wished to remain part of the United Kingdom. It ran for three decades up until the Good Friday peace agreement in 1998.

Even McGuinness's place of birth is a question of politics -- the nationalists call it Derry while the unionists refer to it as Londonderry.

Despite working with the UK government for peace in Ireland, McGuinness remained opposed to British rule in Northern Ireland until the end of his life.

"I am an Irish republican," he told CNN's Nic Robertson. "An Irish republican is someone who believes that the British government should have no part to play in the life of this island. We believe this island should be free."

McGuinness and Britain's Queen Elizabeth II made history in 2012 when they shook hands during her visit to Northern Ireland, a symbol of repairing relations between the United Kingdom and the province. The Queen, whose cousin, the Earl of Mountbatten, was killed by an IRA blast in 1979, sent a private message of condolence to McGuinness's widow, a Buckingham Palace spokesperson told CNN.

McGuinness resigned from his position in January 2017.

high-profile politicians in the UK and Ireland were also quick to offer their condolences. Irish President Michael Higgins said in a statement that McGuinness's passing Manyhigh-profile politicians in the UK and Ireland werealso quick tooffer their condolences. Irish President Michael Higgins said in a statement that McGuinness's passing left a gap that would be "difficult to fill."

"The world of politics and the people across this island will miss the leadership he gave, shown most clearly during the difficult times of the peace process, and his commitment to the values of genuine democracy that he demonstrated," he said.

Former British Prime Minister Tony Blair, who was in Downing Street at the time of the Good Friday Agreement was signed in 1998, said peace in Northern Ireland would not have been possible without McGuinness.

"Once he became the peace maker, he became it wholeheartedly," Blair said in a statement.

Blair added: "I will remember him therefore with immense gratitude for the part he played in the peace process and with genuine affection for the man I came to know and admire for his contribution to peace."

British Prime Minister Theresa May said McGuinness made an "essential and historic contribution to the extraordinary journey of Northern Ireland from conflict to peace."

Kyle Paisley, son of former Northern Ireland First Minister Ian Paisley, who McGuinness served alongside as Deputy First Minister, said on Twitter that McGuinness had shown care towards his father when he was suffering ill health.

"Look back with pleasure on the remarkable year he and my father spent in office together and the great good they did," he wrote. "Will never forget his ongoing care for my father in his ill health."

1/2 Very sorry to hear about the passing of Martin McGuinness.Look back with pleasure on the remarkable year he and my father... — Kyle Paisley (@JCKP1966) March 21, 2017

'Cowardice'

McGuinness's death also drew comment from those who refused to recognize his move from paramilitary to politician.

Norman Tebbit was buried beneath the rubble of the Grand Hotel in Brighton when the IRA bombed the Conservative Party Conference in 1984 in an audacious attempt to assassinate then Prime Minister Margaret Thatcher.

Five people were killed and 34 were injured, including Tebbit's wife, Margaret, who was paralyzed from the waist down when the bomb went off on October 12.

Lord Tebbit, who was then trade secretary, said he was never convinced by McGuinness's move into politics.

Norman Tebbit and former Prime Minister Margaret Thatcher were both in the hotel when it was bombed by the IRA in 1984.

"He (McGuinness) did have an important role in the peace negotiations but that was not because he had changed his mind," Tebbit told CNN.

"He was aware that the Army intelligence services had infiltrated the IRA right up at the top of the Army Council and that the IRA was defeated.

"He had to get something out of defeat because apart from anything else, he would have been charged with the terrorist murder of which he was guilty.

"It was an act of cowardice, not of principle. If anyone thinks that terrorists suddenly become nice Christian gentlemen then that would be a terrible mistake."

Brexit almost delivers for McGuinness

McGuinness may have passed away within striking distance of his most closely held wish -- a unified Ireland.

In the recent UK referendum on Europe, a majority of Northern Ireland voters chose to remain within the European Union

Among their concerns was the state of the border between the Republic of Ireland and Northern Ireland, currently governed by a common travel area which allows free travel between the two countries.

It was a key part of the Northern Ireland peace deal but under the UK government's Brexit white paper, Downing Street only said it would "aim" to protect the open border

McGuinness' Sinn Fein party called for a referendum on Irish unity in the wake of Brexit , which would take Northern Ireland out of the United Kingdom.

Speaking at a news conference in Derry, Michelle O'Neill, Sinn Fein leader in Northern Ireland, paid tribute to an "international statesman."

"He was a man that was recognized as a peacemaker and a man that touched the lives of so many people," she said.