Owen O'Connolly, the informant on this occasion, was one of those ruffling squires or henchmen, who accompanied gentlemen of fortune in that age, to take part in their quarrels, and carry their confidential messages. That he was not an ordinary domestic servant, we may learn from the fact of his carrying a sword, after the custom of the class to which we have assigned him. At this period he was in the service of Sir John Clotworthy, one of the most violent of the Puritan Undertakers, and had conformed to the established religion. Through what recklessness, or ignorance of his true character, he came to be invited by Colonel Hugh McMahon to his lodgings, and there, on the evening of the 22nd, entrusted with a knowledge of next day's plans, we have now no means of deciding. O'Connolly's information, as tendered to the Justices, states that on hearing of the proposed attack on the Castle, he pretended an occasion to withdraw, leaving his sword in McMahon's room to avoid suspicion, and that after jumping over fences and palings, he made his way from the north side of the city to Sir William Parsons at the Castle. Parsons at first discredited the tale, which O'Connolly (who was in liquor) told in a confused and rambling manner, but he finally decided to consult his colleague, Borlase, by whom some of the Council were summoned, the witness's deposition taken down, orders issued to double the guard, and officers despatched, who arrested McMahon at his lodgings. When McMahon came to be examined before the Council, it was already the morning of the 23rd; he boldly avowed his own part in the plot, and declared that what was that day to be done was now beyond the power of man to prevent. He was committed close prisoner to the Castle where he had hoped to command, and search was made for the other leaders in town. Maguire was captured the next morning, and shared McMahon's captivity; but O'Moore, Plunkett, and Byrne succeeded in escaping out of the city. O'Connolly was amply rewarded in lands and money; and we hear of him once afterwards, with the title of Colonel, in the Parliamentary army.
As McMahon had declared to the Justices, the rising was now beyond the power of man to prevent. In Ulster, by stratagem, surprise, or force, the forts of Charlemont and Mountjoy, and the town of Dungannon, were seized on the night of the 22nd by Sir Phelim O'Neil or his lieutenants; on the next day Sir Conor Magennis took the town of Newry, the McMahons possessed themselves of Carrickmacross and Castleblaney, the O'Hanlons Tandragee, while Philip O'Reilly and Roger Maguire razed Cavan and Fermanagh. A proclamation of the northern leaders appeared the same day, dated from Dungannon, setting forth their "true intent and meaning" to be, not hostility to his Majesty the King, "nor to any of his subjects, neither English nor Scotch; but only for the defence and liberty of ourselves and the Irish natives of this kingdom." A more elaborate manifesto appeared shortly afterwards from the pen of Rory O'Moore, in which the oppressions of the Catholics for conscience' sake were detailed, the King's intended "graces" acknowledged, and their frustration by the malice of the Puritan party exhibited: it also endeavoured to show that a common danger threatened the Protestants of the Episcopal Church with Roman Catholics, and asserted in the strongest terms the devotion of the Catholics to the Crown. In the same politic and tolerant spirit, Sir Conor Magennis wrote from Newry on the 25th to the officers commanding at Down. "We are," he wrote, "for our lives and liberties. We desire no blood to be shed, but if you mean to shed our blood, be sure we shall be as ready as you for that purpose." This threat of retaliation, so customary in all wars, was made on the third day of the rising, and refers wholly to future contingencies; the monstrous fictions which were afterwards circulated of a wholesale massacre committed on the 23rd were not as yet invented, nor does any public document or private letter, written in Ireland in the last week of October, or during the first days of November, so much as allude to those tales of blood and horror, afterwards so industriously circulated, and so greedily swallowed.