According to Ptolemy, the inhabitants of this part of the island, and also of the present county of Kildare, were the Cauci, supposed to have been of Belgic-Gaulish extraction. But it is chiefly celebrated as the country of the Byrnes and the O'Tooles, the former of whom occupied the northern and eastern parts, and the latter the south-western. The country of the Byrnes on the western side of the mountains was called the Ranelagh, or Kilconnell, and in Queen Elizabeth's time, Pheagh Mac Hugh's country, from the name of the chief of the Byrnes. Another sept of the Byrnes inhabited the eastern side, bordering on the sea; while the country of the O'Tooles was called Imale, and comprised the mountain regions surrounding the great glen of Imale. The O'Cullans possessed a tract along the northern confines, but they are scarcely mentioned after the Anglo-Norman invasion; and the Danes appear to have had some settlements on the coast. After the arrival of the English, the maritime portions of the county most easy of access were partitioned among the adventurers, and the Byrnes were compelled to retire to the mountains, as also were the O'Tooles, who had previously occupied part of the county of Kildare. On the division into counties by King John, this extensive region was included in that of Dublin; but the septs of the mountains did not acknowledge the English jurisdiction until many centuries after. Secured from successful pursuit by their mountain fastnesses, they waged an incursive warfare against the surrounding English settlements, and more particularly against the citizens of Dublin, of whom, on one occasion, they slaughtered three hundred at Cullen's-wood, where the latter had assembled for recreation at Easter. Besides several fortresses built for private protection, royal castles to keep the natives in check were erected at Newcastle and at Castle Kevin near Annamoe, but with little effect. Piers Gaveston, in the reign of Edw. II., drove back the septs with considerable slaughter into their mountain fastnesses, after which they became so powerful that they were accustomed to make formal treaties with the English authorities. They were, however, so overawed by the first military expedition of Rich. II., that they agreed, with the rest of the native tribes, to evacuate Leinster; but in 1398, after this monarch's return to England with his army, the fulfilment of the agreement was refused; upon which Roger Mortimer, Earl of March, the king's lieutenant, attended by the Earl of Ormond, marched against the septs of Byrne, and drove them from their lands in Wicklow ; but at the very moment of their triumph, while feasts were held and knights created in honour of this success, they were disturbed by the intelligence of a victory gained by the neighbouring sept of O'Toole, who slaughtered a considerable number of the king's forces. The Byrnes retired into Ossory, and there maintained the war with obstinacy; and Mortimer, pursuing them with more courage than circumspection, was surprised, defeated, and slain. About 1402, the septs of Wicklow were severely chastised by the arms of the magistrates of Dublin ; and in later times they sued to become English subjects. In the 34th of Hen. VIII., the Byrnes of the mountains, who had lately sworn allegiance, earnestly desired that their country might be converted into a distinct county, and called the county of Wicklow; but this request was either neglected or refused. When the opponents of the English government had acquired increased strength by fomenting religious dissensions, the celebrated Pheagh Mac Hugh Byrne, in the years 1577, 1578, and 1580, in alliance with several disaffected lords, harassed the English pale; and in the last-named year obtained a sanguinary victory over the lord-deputy's forces at Glendalough, whither they had penetrated with great difficulty. In 1595, on a reverse of fortune, he made his submission at Dublin. In 1596, his sept was defeated by the British troops, after a sharp action; and in the following year, Pheagh Mac Hugh fell in an engagement with the lord-deputy, Sir William Russell. His son Phelim Mac Pheagh was chosen in his place as chief of the Byrnes, and in 1600 made a humble submission to Queen Elizabeth, together with several other Irish toparchs. An expedition was undertaken against him, however, in the same year; but the country was reduced to comparative tranquillity in 1605, in the reign of James I., and during the lieutenancy of Sir Arthur Chichester, by being erected into a county distinct from that of Dublin, under its present name, The Byrnes, in the wars of 1641, united with their neighbours of the same party in the counties of Wexford and Carlow, and extended their ravages to the very walls of Dublin. Notwithstanding the cruelties exercised by Sir Charles Coote in his expedition against them, they maintained their cause until Cromwell, after the siege of Drogheda, marched triumphantly through the county, and reduced every town and fort in it; thus terminating the war in this quarter. In the disturbances of 1798 the county was the scene of many acts of violence, and in the southern part of it several severe conflicts took place. Even after their general suppression, bands of insurgents found a refuge in its mountain recesses, and hence committed extensive depredations, which a large military force was unable to repress. Government at length entered into composition with the principal leaders, in order to restore tranquillity to the country, and cut roads through the wildest districts, and erected barracks at different places in them, which have effected the object proposed, and also tended much to improve the country by facilitating the means of communication through a district previously almost impassable.
Transcribed from A Topographical Dictionary of Ireland, 1840 by Samuel Lewis