‘The present town of Cahir owes its rise to the late Earl of Glengall, and has been enlarged and greatly improved by the present earl. Cahir, however, is of remote antiquity, and it appears that a castle was built here prior to the year 1142 by Connor, King of Thomond; and in the reign of John, Geoffry de Camoell founded an abbey of which there are still some remains. The manor was one of those belonging to the Butler family, and in the reign of Elizabeth the castle was besieged by the Earl of Essex, with the whole of his army, when the garrison, encouraged by the hostilities then waged by the Earl of Desmond, held out for ten days, but was compelled to surrender. In 1647, this fortress was invested by Lord Inchiquin, and, notwithstanding its great strength, surrendered in a few hours after some of its outworks had been gained by the assailants.
Cahir Castle, the extensive old seat of the Butlers, is in the town. It is in good preservation and occupying the summit of an isolated rock, which rises over the left bank of the Suir, is a highly interesting and picturesque object.’
From A Handbook for Travellers in Ireland by James Fraser, 1844. 

‘The chief historical events connected with the castle were the sieges of it by the Earl of Essex, in the reign of Queen Elizabeth, and by Lord Inchiquin, in that of Charles I. The readers of English history are familiar with the unhappy expedition of Essex to Ireland, which was greatly promoted by his powerful enemies at court, as certain to end unfortunately, and thus as certainly to break his influence with the queen. Former viceroys and commanders in Ireland had suffered disaster upon disaster; and by the Battle of the Blackwater, in 1598, the English forces were reduced to the lowest ebb. Essex landed with an army of more than 20,000 men, the largest force, according to the Four Masters, sent to Ireland by the English since the invasion of Strongbow. But Essex was no more successful than his predecessors. His orders were, in the first place, to reduce the rebels in Ulster, and to put strong garrisons into their forts; but, instead of this, he marched into Munster and laid siege to Cahir Castle. He invested it with 7,000 foot and 1,200 horse; but the Earl of Desmond and Redmond Burk came to its relief, and Essex found himself unable to reduce it till he had sent to Waterford for heavy ordnance. On the tenth day of the siege, being the 20th of May, 1599, the castle was surrendered to the Earl of Essex and the Queen. But the surrender of the castle was of no real advantage. He made, indeed, capture of the rebels’ cattle in those parts, and drove the rebels themselves into the woods and mountains; but, as fast as he retired again towards Dublin, these rebels came out from their retreats and followed on his track, harassing his rear, so that his return was rather like a rout than the march of a conqueror. The disasters which befell him on this journey completed his ruin.’
From Ruined Abbeys and Castles of Great Britain and Ireland by William Howitt, 1864. 

‘During the troubles which followed on the rebellion of 1641, Cahir Castle was taken for the Parliament, by surrender, in the beginning of August 1647 by Lord Inchiquin; and it was again taken in February 1650 by Cromwell himself, the garrison receiving honourable conditions. The reputation which the castle had at this period as a place of strength will appear from the account of its surrender as given in the manuscripts of Mr Cliffe, secretary to General Ireton, published by Borlase. After observing that Cromwell did not deem it prudent to attempt the taking of Clonmel till towards summer, he adds that he “drew his army before a very considerable castle called Cahir Castle, not very far from Clonmel, a place then possessed by one Captain Mathews, who was but a little before married to the Lady Cahir, and had in it a considerable number of men to defend it; the general drew his men before it, and for the better terror in the business brought some cannon with him likewise, there being a great report of the strength of the place, and a story told the general, that the Earl of Essex in Queen Elizabeth’s time, lay seven or eight weeks before it and could not take it. He was notwithstanding then resolved to attempt the taking of it, and in order thereunto, sent them this thundering summons:-
“Sir – Having brought my army and my cannon near this place, according to my usual manner in summoning places, I thought fit to offer you terms honourable for soldiers, that you may march away with your baggage, arms and colours, free from injuries or violence; but if I be, notwithstanding, necessitated to bend my cannon upon you, you must expect what is usual in such cases. To avoid blood, this is offered to you by,
Your Servant,
O. Cromwell”
Notwithstanding the strength of the place, and the unseasonableness of the time of the year, this summons struck such a terror in the garrison, that the same day the governor, Captain Mathews, immediately came to the general and agreed to the surrender.’
From The Irish Penny Journal, Volume 1, No.33, February 1841.