After some skirmishing the fortress was closely besieged in 1642, and of the nine hundred people in town and castle many died of starvation, while others were reduced to eating dogs and cats.
At length the Earl of Ormond sent a detachment to its relief, but early the next year General Preston approached the stronghold with artillery and troops.
He reconnoitred the town from Drumbawn Hill, and after firing a few shots encamped in the neighbouring woods. On the second day of the siege he sent a messenger to the Governor to inquire if he held the castle for the King or the Parliament, and asking to be allowed to garrison it for his Majesty’s use.
Sir William replied that he had not heard of any difference from his Majesty or from the Parliament, and that he held his commission as Governor of Ely O’Carroll.
At this reply Preston entrenched, and next day began to bombard the fortress in earnest. The following night a mine was commenced under the direction of a mason who had been employed in the castle. The garrison, hearing the noise, fired on the sappers, but the darkness prevented the shot taking effect, and in the morning they were underground.
Preston continued firing, and destroyed much of the wall and one of the flankers. He said he would break down the fortress about the Governor’s ears. Some of the balls found in the masonry weighed 9 lbs. each.
Under cover of a parley the defenders of the mill were withdrawn, and the garrison held out for two days after they had been undermined. A conference was held and the besieged were granted honourable terms.
Lord Castlehaven conducted them to Athy. They numbered about eight hundred men, women, and children.
Preston seems to have retained possession of Birr until 1645, when for five years it was held by the Confederate Catholic forces.
It was taken from the Irish in 1650 by General Ireton, the enemy having burnt it before retreating. The Marquis of Clanrickarde tried to retake it in vain.