Since the inception of the Irish Volunteers, the Irish Republican Brotherhood had planned on using the organisation to stage an armed rebellion, with the goal of separating Ireland from the United Kingdom and establishing a republic. The entry of the UK into the First World War was, in their view, a perfect opportunity to do so. With the co-operation of James Connolly and the Irish Citizen Army, a secret council of IRB officials planned a general rising on Easter, 1916. On the Wednesday before, they presented MacNeill with a letter, allegedly stolen from high-ranking British staff in Dublin Castle, indicating that the British were going to arrest him and all the other nationalist leaders. Unbeknownst to MacNeill, the letter, called the Castle Document, was a forgery. When MacNeill learned about the IRB’s plans, and when he was informed that Roger Casement was about to land in Co Kerry with a shipment of German arms, he was reluctantly persuaded to go along with them, believing British action was now imminent and mobilisation of the Volunteers would, at this stage, be a defensive act. However, on learning of the arrest of Casement, and the loss of the promised German arms, MacNeill countermanded the order for the Rising in print, severely reducing the number of volunteers who reported for duty on the day of the Easter Rising. Pádraig Pearse, James Connolly, and the others all agreed that the rising would go ahead anyway, and it began on Easter Monday, 24 April 1916. The rising lasted less than a week, and after the surrender of the rebels, MacNeill was arrested and sentenced to life imprisonment, though he had taken no part in the insurrection.

Image colourised by 1916 Revolution in Colour