Martin McGuinness, a former Irish Republican Army commander and deputy first minister of Northern Ireland who was a key figure throughout five decades of conflict and peace, has died aged 66, his party, Sinn Fein, said.

McGuinness, whose journey from street fighter to peacemaker began in the 1970s during Northern Ireland's "Troubles", had bowed out of politics several months earlier than planned in January due to an undisclosed illness.

"Throughout his life Martin showed great determination, dignity and humility, and it was no different during his short illness," Sinn Fein President Gerry Adams said in a statement.

"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."

McGuinness abandoned a butcher's apprenticeship in 1970 to join the Irish Republican Army in a bloody campaign to end British rule in Northern Ireland. He played a key role in both the start and the end of the province's 30-year sectarian conflict, in which some 3,600 people were killed.

File photo dated August 1985 of Noraid leader Martin Galvin (right) standing alongside Martin McGuinness, as the two men acted as pall-bearers at the Londonderry funeral of IRA man Charles English.

WAR AND PEACE

McGuinness later admitted he was second-in-command in Londonderry on "Bloody Sunday" - the day in 1972 when British troops in the city killed 14 unarmed marchers, ushering in the most intense phase of the Troubles.

In the 1980s, McGuinness emerged alongside Adams as a key architect in the electoral rise of Sinn Fein, the IRA's political ally, and became the party's chief negotiator in peace talks that led to the 1998 peace deal.

McGuinness had been deputy first minister for a decade before quitting in January in protest at First Minister Arlene Foster of the Democratic Unionist Party's (DUP) handling of a controversial green-energy scheme. His resignation led to the collapse of the power-sharing government.

"History will record differing views and opinions on the role Martin McGuinness played throughout the recent and not so recent past but history will also show that his contribution to the political and peace process was significant," said Foster, who survived an IRA bomb attack on a school bus at the age of 17.

"In recent years his contribution helped build the relative peace we now enjoy."

Shortly after his retirement, the party achieved a major electoral breakthrough in elections to the regional assembly, coming within one seat of the Democratic Unionist Party and depriving the pro-British political camp of an overall majority for the first time since the partition of Ireland in 1921.

Irish President Michael D. Higgins said McGuinness played an "immense contribution" to the advancement of peace and reconciliation in Northern Ireland – a contribution he said was rightly recognised across all shades of opinion.

"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," Higgins said in a statement.

"His death leaves a gap that will be difficult to fill. May he rest in peace."