Eoin MacNéill was a scholar, revolutionary and politician from Glenarm in County Antrim.
A champion of the Irish language, he founded the Gaelic League, an organisation which aimed to encourage the use of Irish in everyday life, in 1893.
He was a founding member of the Irish Volunteers, a group that played a major role in the Easter Rising, in 1913 and became its chief of staff.
But, he has gone down in history as the man who tried to stop the Easter Rising.
When the Irish Volunteers were told to join the rebellion in April 1916, MacNéill issued a countermand order that said "volunteers completely deceived".
He was later arrested and jailed for his actions.
The 11-word countermand was sold for over £24,000 two years ago and is considered one of the most important documents in Irish history.
A new Irish language documentary on the life of MacNéill gives an insight into this moment, long considered one of the most controversial of the Easter Rising.
The programme, called 'Eoin MacNéill-Fear Dearmadta 1916' (the Forgotten Man of 1916) uses dramatic reconstructions from his unpublished memoir, along with interviews with historians and experts, to examine this pivotal moment in Irish history.
It also features a personal account from MacNéill's grandson Michael McDowell, the former tanaiste (Irish deputy prime minister).
Today, opinion remains divided about MacNéill. Is he a hero or hate-figure?
The director of the documentary, Damian McCann, said MacNéill's much maligned decision was one of the most interesting aspects of MacNéill's story.
"There is a deep divide in opinion about MacNéill, but his story cannot be told simply.
"It is not black and white, and, in fact, MacNéill's dramatic life reflects the complex history of this period.
"The great themes of life are examined in this documentary - conflict, friendship and betrayal."
Prof Diarmaid Ferriter added: "We cannot reduce the options or the choices that were facing people like MacNéill in such crude terms, that they had either to be on one side of a debate or another side.
"There was a hell of a lot in between."
MacNéill was a professor of history at University College Dublin (UCD), and one of the leaders of the Irish language revival movement.
He published an article called 'The North Began' in An Claidheamh Soluis, the newspaper of the Gaelic League, calling for the formation of an Irish Volunteer force to emulate the Ulster Volunteer Force established in January 1913.
The Irish Volunteers were formed later that year, with MacNéill as commander-in-chief.
MacNéill had no role in planning the Easter Rising, which was carried out by members of the Irish Republican Brotherhood (IRB) - an organisation dedicated to the establishment of an independent democratic republic in Ireland.
When he learned about their plans, and that a ship loaded with arms from Germany to assist the rebellion had been sunk, he countermanded orders for a rebellion, placing a news advertisement advising Irish Volunteers not to take part.
This decision delaying the Easter Rising for a day, and largely frustrated it outside Dublin.
MacNéill was arrested after the suppression of the rebellion. He was court-martialled and sentenced to life imprisonment.
He was released from prison in 1917.
Eoin MacNéill - Fear Dearmadta 1916 is broadcast on BBC Two Northern Ireland on Sunday 27 March at 22:00 GMT.