He made his way with a small band of armed retainers to St. Mary’s Abbey, on the north side of the Liffey, and then outside the boundaries of the city of Dublin. The Council was assembled there, when Thomas rushed into the room and, after a defiant speech, in which he accused King Henry of his father’s murder, he flung the Sword of State, the emblem of his official authority, on the table, and renounced his allegiance. Cromer, the Primate, ventured to remonstrate with him, but hardly had he finished his exhortation, when one of the young lad’s followers, Niall O’Kennedy, the harper, struck his instrument, and began to sing of the past glories of the Geraldines, and to urge their descendant to emulate their valour.

This for Thomas decided the question in favour of rebellion.   With a few rough words he silenced the Archbishop, and before the members of the council  had  fully  recovered  from  their  amazement  at  this extraordinary scene, he and his retainers had mounted their horses and  galloped off.
In July (1534) Thomas appeared before Dublin, and the citizens, feeling themselves not strong enough for resistance, allowed his troops to enter the city and lay siege to the Castle. Amongst those who had taken refuge in the Castle at the beginning of the rebellion was John Alen, Archbishop of Dublin.

He had been on the Council one of the chief enemies of Garrett Og, and now dreaded his son’s vengeance, in case the Castle were captured. Before the siege began, he endeavoured to escape by sea, but his boat was driven ashore at Clontarf, and he himself was dragged into the presence of Thomas. The young lord regarded him with contemptuous pity, and turned away, with the remark, ” beir uaim an bodach,”*but his followers, wilfully misinterpreting this order, proceeded to murder the Archbishop in cold blood.

This crime, although Thomas was not apparently himself responsible for it, had an important effect in hastening the ruin of his cause. It alienated the sympathies of his supporters at home and abroad, and, most fatal of all, produced a sentence of excommunication from the Pope.

All the summer and autumn fighting went on in and around the Pale. The citizens of Dublin rose and expelled the besiegers of the Castle, and the Geraldine party then laid siege to the city itself. Thomas made frequent raids, mostly into the territory of the Butlers, while they, on their side, ravaged the Geraldine lands.
So far the rebels had only been opposed by what might be described as private armies, the feudal forces of certain of the Pale nobility. As yet no help had come from England, though Sir William Skeffington had been appointed Deputy several months before, and a large army had been gathered together for an Irish campaign.

 It was not until October (1534) that this army crossed the Channel, and when it arrived it accomplished nothing for a considerable time. After various useless marches through the country, the English forces took up their quarters in Dublin, where they passed the winter. Outside, Lord Thomas raided the country at his pleasure, and burnt the towns of Trim and Dunboyne.
In spite of this apparent success, the Geraldine party was beginning to fall to pieces. Numbers of Thomas’ adherents went over to the side of the Crown, and the excommunication, promulgated against their leader in November, increased the number of these defections.

It must have become apparent to the young Geraldine—his father had died in the Tower in December, 1534, so he was now Earl of Kildare —that, as Henry VIII showed no inclination to come to terms with him, he could not, without foreign aid, long maintain the struggle. His attempts to obtain such aid met with no success. Probably the Continental monarchs concluded, from the information of theii agents, that the rebellion was little likely to develop into anything sufficiently important to embarrass Henry VIII. So they turned a deaf ear to his entreaties. Nor could he obtain from the Pope, before whom he endeavoured in vain to pose as the champion of Catholicity against the schismatic Henry VIII, a reversal of the decree of excommunication pronounced against him.
In the spring of 1535 Skeffington bestirred himself, and marching from Dublin, laid siege to the Geraldines’ castle of Maynooth. This fortress was reckoned to be too strong to be reduced by any method but the tedious one of blockade. The shattering power of the improved artillery had not, however, been taken into consideration, though some kind of artillery had been in occasional use in Ireland for a good while
previously.

Eight days sufficed to make such breaches in the walls that the garrison were obliged to surrender. All were executed. The fall of Maynooth should have served as a warning to the Pale nobility and the chiefs that a new era in warfare had set in, and that the chances against small rebellions had enormously increased.