This district, so far as can be collected from Ptolemy, Was the habitation of the Brigantes and Cauci, or, according to Whitaker, of the Coriundi. Afterwards it formed the northern part of the principality of Hy Kinselagh, and was distinguished by the name of Hy Cabanagh. and Hy Drone, in later times it was called Catherlough. It is noticed in the earliest period of Irish history as the scene of contention between Conmal, son of Heber, and grandson of Milesius, and a descendant of Heremon, the latter of whom was defeated at Leighlin. When Con of the Hundred Battles, who reigned about the middle of the second century, divided the island into two jurisdictions, Dinrigh or Dewa Slaney, between Carlow and Leighlin, and Naas in Kildare, were made the sites of the royal palaces of the kingdom of Leinster. No traces of ruins, however, now exist to confirm the truth of this traditionary record, with respect to the former of those places. The synod of the clergy held about the year 630, to decide on the proper time for the celebration of Easter, met at St. Gobhan's abbey, in Old Leighlin; and about the same time the bishoprick, which takes its name from that place, was founded. That the county shared with the other parts of the island in the devastations committed by the Danes, during the ninth and tenth centuries, appears from the fact that the rich abbey of Achadfinglas was plundered by them in 864. The year 908 was distinguished by a decisive battle between the people of Leinster and those of Munster, the latter headed by Cormac Mac-Cuillenan, better known as the writer of the Psalter of Cashel than by his political or military acts: the scene of this battle was at Moyalbe, supposed by O'Halloran and Lanigan to be somewhere in the vicinity of Ballymoon, in this county; the Munster men were defeated, and Cormac, with many of his nobles and officers, and six thousand of his best soldiers, slain. In the same century, the monastery of St. Mullins was plundered by the Danes, and Leighlin was three times taken by the people of Ossory. After the arrival of the English, it appears that some of the petty chieftains of the district refused to join in the alliance formed by Dermot Mac Murrough, their king, with the Welsh invaders. For when Strongbow, after having dispersed the numerous army with which Roderic, King of Ireland, had invested Dublin, marched southward to relieve Fitz-Stephen, then blocked up in Carrig castle, near Wexford, he was assailed during his passage through Hy Drone by O'Ryan, the lord of the country, with such impetuosity that victory remained doubtful, until the death of the Irish leader turned the scale in favour of the invaders. It was in this battle that Strongbow is said to have hewn his son, a youth about fifteen years of age, in two, for deserting his post during the engagement. The importance attached by the conquerors to the possession of the territory thus acquired is evident from the fact that, within a few years after, the castles of Carlow, Leighlin, and Tullow, were erected by Hugh de Lacy, then lord-deputy. After the death of William, Earl-Marshal, to whom nearly the whole of Leinster belonged in right of his wife Isabel, daughter of Strongbow by Eva, princess of Leinster and heiress of Dermot Mac Murrough, this vast estate was divided among his five daughters; and the palatinate of Carlow, which had been previously made one of the twelve counties into which King John divided all those parts of Ireland that acknowledged his government devolved by marriage on Hugh le Bigod, Earl of Norfolk, who thus became earl-marshal and lord of Carlow, in right of his wife Maud, eldest daughter of the deceased. For many subsequent years the English kept possession of these border districts by a very frail tenure. At the close of the thirteenth century, Old Leighlin was burnt in an incursion of the people of the neighbouring territory of Slieumargy, which was then considered to be part of the county; and, at the commencement of the next century, it appears that the owners of this princely estate, the palatinate of Carlow, having also large possessions in England, paid but little attention to its interests. Residing in another country, and finding their income from this quarter diminishing, in consequence of the mismanagement of their deputies and the disturbed state of the country, they had recourse to a remedy, which, however effectual at first, ultimately proved destructive to their interests in this quarter. They retained one of the Kavanaghs, the descendants of Mac Murrough, and, though illegitimate, the inheritor of his hereditary rights, as a kind of military agent, to supply by the sword the deficiencies of the law. Kavanagh, thus placed in a situation peculiarly tempting to a turbulent and ambitious character, soon broke the connection, and seized upon a great portion of Carlow and Wexford, as belonging to him of right : he further assumed the regal title of Mac Murrough, and strengthened his newly acquired power by an alliance with the O'Byrnes and O'Tooles of the neighbouring mountainous district of Wicklow. In 1316, Sir Edmund Butler, lord-justice, defeated Mac Murrough near Ballylethan; and the same year was marked by the incursion of Edward Bruce into the southern counties. But though the invader passed through Castledermot and Tullow, in his progress southward, he made no impression on this county; and, that it still continued subject in a great degree to the sway of the Kavanaghs may be inferred from the circumstance that, in 1323, Donnell Mac Arthur Mac Murrough, "a slip of the royal family," as Campion calls him, raised forces and displayed his banner within two miles of the city of Dublin. He paid dearly, however, for his temerity, being defeated by a party of the garrison. O'Nolan, dynast of Forth barony, and twenty-five of his followers were killed; and Mac Murrough's life was spared only on payment of £200, a large sum in those days; after remaining six years immured in Dublin castle, he at length contrived to effect his escape through the connivance of his keeper.
Transcribed from A Topographical Dictionary of Ireland, 1840 by Samuel Lewis