‘Whilst there is scarcely an old castle, abbey or ruin of any pretensions throughout the length and breadth of Green Erin, since the introduction of cheap literature, that has not been over and over described till we have naturally began to tire of their repetition, is it not strange that none of these popular writers have as yet attempted a description of the many remnants of antiquity abounding in Clonmore in the County of Carlow, particularly its venerable castle? From Ben-Hadir to Ben-Urris, from Donaghadee to Dingle, there is scarcely a place whose particular beauties have not at some time been duly chronicled; guide books in variety too, have been given to the public of Antrim and Cork, of Wicklow and Kerry, and every picturesque locality round our island; yet amongst them all, poor Clonmore – but a few hours drive (only thirty-five Irish miles) from the metropolis – has been completely unheeded and neglected. None of the Penny Journals published by Folds or Coldwell, Hardy, or Gunn and Cameron have even mentioned its name! Grose, in his Antiquities of Ireland, published in 1791-3, has given two neat views of the castle, which the Author of these pages is happy to say he has a few copies of; but then Dr. Ledwich, who furnished the descriptive portion of that work, on account of Grose’s premature death, has dismissed the subject in a few lines. Lewis’ Topographical Dictionary and Ryan’s History of the County Carlow, both of their accounts of the castle and its antiquities are meagre enough, and incorrect in some particulars, which I intend to point out as I proceed; and, with the exception of these three publications, no recent writer that I could come at has ever favoured us with even a single line on the subject.’
From The Antiquities and History of Cluain-Mor-Maedhoc, Now Clonmore by John MacCall (1862)

Clonmore Castle: The spacious piece of antiquity of this place is situate near Hacketstown, and in the barony of Rathvilly. In shape it is square; one hundred and seventy feet by the same. The castle has towers at each angle, and is surrounded by a fosse, of about twenty feet in depth. The walls are five feet thick; and the narrow, stone-cased windows were obviously furnished with iron bars. One of the side walls has disappeared, but the other three are in good preservation and, if unassailed by the Gothic hands of man, will probably resist the tooth of Time for ages to come. The demolished wall, was no doubt removed in order to procure ingress to two or three cabins and their appurtenances, which classically ornament the interior. Indeed, I have been credibly informed, that part of the window-cases now serve the very ignoble purpose of forming part of the materials of some pig-sties! But such desecration of ancient works of art, by the unthinking and ignorant, is not at all an uncommon circumstance in this country.’
From The History And Antiquities Of The County Of Carlow by John Ryan (1833) 

‘Cromwell’s army landed in Dublin in August 1649 and in 1650 the Cromwellian colonels, Hewson and Reynolds, captured the castle and ordered it to be slighted so as to make it indefensible, reducing the castle to the ruins that may be seen today. The fortress was both strong and large, square in plan, with high curtain walls defended with a tower at each corner. Although the Parliamentarians destroyed the gatehouse, extensive ruins indicating various halls and chambers remain. The northeast tower, known as the Six Windows, is still well preserved complete with a gargoyle known as “the pooka’s head.” Patrick Wall was granted the castle of Ballynekill (Clonmore) following the restoration of Charles II, together with 69 acres and 1 rood. In 1697, following the Williamite Settlement, what remained of the castle passed into the hands of Ralph Howard of Dublin. He was created Baron Clonmore in 1776, elevated to Viscount Wicklow in 1785, and his son Robert was made Earl of Wicklow in 1793. Clonmore was still in the hands of the Howard family in 1823, but then, around 1900, it passed to the Stopford family, Earls of Courtown.’
From The Byrnes and the O’Byrnes, Vol. II by Daniel Byrne-Rothwell (2010)