In early April, Pearse issued orders to the Irish Volunteers for three days of "parades and manoeuvres" beginning on Easter Sunday. He had the authority to do this, as the Volunteers' Director of Organisation. The idea was that IRB members within the organisation would know these were orders to begin the rising, while men such as MacNeill and the British authorities would take it at face value.
On 9 April, the German Navy dispatched the SS Libau for County Kerry, disguised as the Norwegian ship Aud. It was loaded with 20,000 rifles, one million rounds of ammunition, and explosives. Casement also left for Ireland aboard the German submarine U-19. He was disappointed with the level of support offered by the Germans and he intended to stop or at least postpone the rising.
On Wednesday 19 April, Alderman Tom Kelly, a Sinn Féin member of Dublin Corporation, read out at a meeting of the corporation a document purportedly leaked from Dublin Castle, detailing plans by the British authorities to shortly arrest leaders of the Irish Volunteers, Sinn Féin and the Gaelic League, and occupy their premises. Although the British authorities said the "Castle Document" was fake, MacNeill ordered the Volunteers to prepare to resist. Unbeknownst to MacNeill, the document had been forged by the Military Council to persuade moderates of the need for their planned uprising. It was an edited version of a real document outlining British plans in the event of conscription. That same day, the Military Council informed senior Volunteer officers that the rising would begin on Easter Sunday. However, it chose not to inform the rank-and-file, or moderates such as MacNeill, until the last minute.
The following day, MacNeill got wind that a rising was about to be launched and threatened to do everything he could to prevent it, short of informing the British. MacNeill was briefly persuaded to go along with some sort of action when Mac Diarmada revealed to him that a German arms shipment was about to land in County Kerry. MacNeill believed that when the British learned of the shipment they would immediately suppress the Volunteers, thus the Volunteers would be justified in taking defensive action, including the planned manoeuvres.
The Aud and the U-19 reached the coast of Kerry on Good Friday, 21 April. This was earlier than the Volunteers expected and so none were there to meet the vessels. The Royal Navy had known about the arms shipment and intercepted the Aud, prompting the captain to scuttle the ship. Furthermore, Casement was captured shortly after he landed at Banna Strand.
When MacNeill learned from Volunteer Patrick Whelan that the arms shipment had been lost, he reverted to his original position. With the support of other leaders of like mind, notably Bulmer Hobson and The O'Rahilly, he issued a countermand to all Volunteers, cancelling all actions for Sunday. This countermanding order was relayed to Volunteer officers and printed in the Sunday morning newspapers. It succeeded only in delaying the rising for a day, although it greatly reduced the number of Volunteers who turned out.
British Naval Intelligence had been aware of the arms shipment, Casement's return, and the Easter date for the rising through radio messages between Germany and its embassy in the United States that were intercepted by the Royal Navy and deciphered in Room 40 of the Admiralty. The information was passed to the Under-Secretary for Ireland, Sir Matthew Nathan, on 17 April, but without revealing its source and Nathan was doubtful about its accuracy. When news reached Dublin of the capture of the Aud and the arrest of Casement, Nathan conferred with the Lord Lieutenant, Lord Wimborne. Nathan proposed to raid Liberty Hall, headquarters of the Citizen Army, and Volunteer properties at Father Matthew Park and at Kimmage, but Wimborne insisted on wholesale arrests of the leaders. It was decided to postpone action until after Easter Monday, and in the meantime, Nathan telegraphed the Chief Secretary, Augustine Birrell, in London seeking his approval. By the time Birrell cabled his reply authorising the action, at noon on Monday 24 April 1916, the Rising had already begun.
On the morning of Easter Sunday, 23 April, the Military Council met at Liberty Hall to discuss what to do in light of MacNeill's countermanding order. They decided that the Rising would go ahead the following day, Easter Monday, and that the Irish Volunteers and Irish Citizen Army would go into action as the 'Army of the Irish Republic'. They elected Pearse as president of the Irish Republic, and also as Commander-in-Chief of the army; Connolly became Commandant of the Dublin Brigade. Messengers were then sent to all units informing them of the new orders.