Tradition states that the Earl of Essex frequently stayed in the castle, and some of his letters to the Queen are dated from Athlone.
The O’Conor Don was imprisoned in the fortress in 1570 while Sir E. Fitton was constable, as a hostage for the good behaviour of his sept. Some of his followers, however, brought a “cot” under the castle walls, into which the captive stepped, and so escaped.
In 1585-86 it is described as being a fitter residence for the Chief Commissioner of Connaught than the Lord Deputy in the following words:—“That the castle is conveniently furnished with buildings and other necessaries fit for the said Commissioner, but far too mean for the Lord Deputy and the train that must follow the state.”
It was ordered to be garrisoned in 1599, and the following year it was to be entrusted to none but a “sound Englishman.” In 1606 it was repaired and added to.
Two years later it was seized by the Earls Tyrone and Tyrconnell. It passed again to the Crown, and the Earl of Clanricard was constable in 1610.
Thirteen years later it was repaired, and a curious tax is mentioned with regard to the operations, which is, that the sept of Kellyes was bound to supply three hundred labourers yearly for work in the fortress.
The Court of Claims sat in the castle during the Commonwealth.
In 1682 Sir H. Piers writes of it:—“In the centre of the castle is a high raised tower which overlooketh the walls and country round about. On the side that faceth the river are rooms and apartments which served always for the habitation of the Lord President of Connaught and Governor of the castle, the middle castle being the storehouse for ammunition and warlike provisions of all sorts.”
After the battle of the Boyne in 1690, Lieut.-General Douglas, with ten regiments of infantry, three of horse, two of dragoons, twelve field-pieces, and two small mortars, endeavoured to take possession of Athlone. The bridge across the Shannon was broken, and he erected his batteries on the Leinster side of the river.
He continued the cannonading for eight days, but his powder running short he was obliged to retire. In his despatch he stated he had done his best, and that it was his opinion Athlone Castle was “of the greatest importance of any in Ireland.”