In May, 1641, Captain Neil O'Neil arrived from the Netherlands with an urgent request from John, Earl of Tyrone, to all his clansmen to prepare for a general insurrection. He also brought them the cheering news that Cardinal Richelieu—then at the summit of his greatness—had promised the exiles arms, money, and means of transport. He was sent back, almost immediately, with the reply of Sir Phelim, O'Moore and their friends, that they would be prepared to take the field a few days before or after the festival of All Hallows—the 1st of November. The death of Earl John, the last surviving son of the illustrious Tyrone, shortly afterwards, though it grieved the Confederates, wrought no change in their plans. In his cousin-germain, the distinguished defender of Arras, they reposed equal confidence, and their confidence could not have been more worthily bestowed.
The plan agreed upon by the Confederates included four main features. I. A rising after the harvest was gathered in, and a campaign during the winter months, when supplies from England were most difficult to be obtained by their enemies. II. A simultaneous attack on one and the same day or night on all the fortresses within reach of their friends. III. To surprise the Castle of Dublin, which was said to contain arms for 12,000 men. IV. Aid in officers, munitions, and money from abroad. All the details of this project were carried successfully into effect, except the seizure of Dublin Castle—the most difficult as it would have been the most decisive blow to strike.
Towards the end of August, a meeting of those who could most conveniently attend was held in Dublin. There were present O'Moore and Maguire, of the civilians, and Colonels Plunkett, Byrne, and McMahon of the army. At this meeting the last week of October, or first of November, was fixed upon as the time to rise; subsequently Saturday, the 23rd of the first named month, a market day in the capital was selected. The northern movements were to be arranged with Sir Phelim O'Neil, while McMahon, Plunkett, and Byrne, with 200 picked men, were to surprise the Castle guard—consisting of only a few pensioners and 40 halbediers—turn the guns upon the city to intimidate the Puritan party, and thus make sure of Dublin; O'Moore, Lord Maguire, and other civilians, were to be in town, in order to direct the next steps to be taken. As the day approached, the arrangements went on with perfect secrecy but with perfect success. On the 22nd of October half the chosen band were in waiting, and the remainder were expected in during the night. Some hundreds of persons, in and about Dublin, and many thousands throughout the country, must have been in possession of that momentous secret, yet it was by the mere accident of trusting a drunken dependent out of sight, that the first knowledge of the plot was conveyed to the Lords Justices on the very eve of its execution.