As war raged around them the people of Dublin looked skyward to see an unfamiliar flag flying from the roof of the GPO.
For centuries the green flag with the yellow harp was the recognised Irish standard but on Easter Monday 1916 the green, white and orange flag was hoisted aloft as a signal of both national freedom and unity.
We also know that a green flag of the 'Irish Republic' was flown over the GPO also.
The emergence of the 'new' flag of unity was seen as a major statement by the Republican movement - a new flag for a new, strong, unified nation.
The making of the 1916 flag was the responsibility of Sean MacDiarmada - secretary of the Irish Republican Brotherhood.
It was secretly made from Irish linen with three sections or 'fields', coloured 'green, white and orange'.
However it's claimed the drapers MacDiarmada commissioned for the job made an error and got their colours mixed up.
When the 29"x 63" flag arrived at the GPO, the three fields had to be hastily unstitched, rearranged and re-stitched.
Shortly before the Proclamation of the Republic was read out, Willie Pearse, the younger brother of Patrick Pearse, made his way to the rooftop of the GPO with two flags in his hand.
He handed the green flag with the words 'Irish Republic' on it to the Argentinian-born Eamonn Bulfin to hoist up the flag pole while it's said the tri-colour was hoisted by a member of the Kimmage Exile Unit.
It's unclear who hoisted the tricolour up the flagpole but the majority of historians feel Quartermaster General Michael Staines may have carried out the task.
Later that day, James Connolly dictated a message to his soldiers in which he told them, 'For the first time in 700 years the flag of a free Ireland floats triumphantly in Dublin City.'
Since Thomas Francis Meagher first unveiled the flag in Waterford some 68-years earlier the green, white and orange design had laid dormant but now as it flew over the GPO it captured the banner of the new revolutionary Ireland and was quickly acclaimed throughout the country as the national flag.
As British forces stormed the GPO it was thought the flags were lost however it later transpired that a Sergeant Tommy Davis from Lisburn in Co. Antrim, who served with the British Army's Royal Dublin Fusiliers, stashed the flag during a clear-up of the city after the Easter Rising.
Three months later, Davis was invalided out of the army at the Somme and returned home to Lisburn where he was looked after by Dr George St George. In lieu of payment, he gifted the flag to the doctor who, upon his death in 1922, passed it on to his son-in-law, Captain Samuel Waring, of Kells, Co. Meath.
In 1951, Captain Waring presented the flag to a neighbour, the son of John Sweetman, co-founder and sometime president of Sinn Féin. The flag, billed as the first flag of the Irish Republic, was put up for auction in 2010 for an asking price of between €500-700,000 but did not sell.
It's presently on long term loan to the American-Irish Historical Society in New York and many feel it should be returned to Ireland for the commemoration of the Easter Rising next year.
A note from Sgt Davis, which accompanies the flag reads: 'Captured by British Troops at GPO Dublin, April 1916, and given to Dr George St George by an old war veteran, Sgt Davis.'
Since the flag celebrated unity it was quickly adopted by all and continued to be recognised by official usage during the period 1922-1937, when its position as the national flag was formally confirmed by the Constitution of 1937, Article 7 of which states: 'The national flag is the tricolour of green, white and orange.'