During the fifteenth century most of Wicklow was in the hands of the O'Byrnes and the O'Tooles.
Developments in both weaponry and military strategy in the three centuries since the arrival of the Normans had largely nullified the defensive capacity of traditional Irish structures such as the ringfort or rath. Gaelic chiefs throughout the country had replaced traditional defences with stone-built tower-houses in order to increase security. For some reason, however, traditional raths survived in Wicklow - the O'Byrnes still relied on them in Glenmalure and Ballinacor.
In an effort to bring the Gaelic rebels in the uplands under control, martial law was established in rebel areas in the 1560's, and military garrisons were established. The presence of such garrisons was an immediate threat to the power and dominance of the Gaelic clans and, unsurprisingly, the clans moved to undermine the government's initiatives.
The O'Byrnes were causing increasing trouble as the 1560s progressed. In the spring of 1572, Robert Browne, a wealthy Wexford landlord, and a firm government supporter, was murdered. The Dublin authorities came under pressure from Queen Elizabeth to act. Fiach (or Feagh) McHugh-O'Byrne was suspected, although in fact, he was innocent. Government overreaction fuelled a full-scale revolt through south Wicklow in the latter half of 1572.
In 1577, leaders of the most important Gaelic clan in Leinster, the O'Mores of King's and Queen's Counties, were invited to muster for military service. Arriving to do their duty, government troops turned on them, massacring more than forty of the clan-leaders, in an act which deeply shocked the remaining Gaelic leaders. A revolt broke out but was quickly quashed. Spurred on, the government repeatedly and ruthlessly attacked Gaelic strongholds. By October 1578 many Gaelic leaders were left with no option but to submit to the authority of the crown. Bitterness and an uneasy peace prevailed.
Following the succession of outrages Fiach McHugh began to muster troops and within a short time had established an army of up to 700 soldiers. With his enlarged army, Feagh was now the most powerful clan-leader in Leinster. A major revolt was brewing.
In early 1580 Fiach commenced a revolt, attacking north Wexford, Carlow and Newcastle. Despite Fiach's forces being routed at Glenmalure, the revolt grew, and Fiach continued to harry government locations. In 1581 Grey sent a substantial force into the mountains with instructions to bring order to the area. Ruthless means, including wanton killing of civilians and scorched earth policies, were pursued, aimed at sapping the zeal of the rebels. The Leinster revolt ended when Feagh submitted in 1582.
By 1591, the area having remained quiet for the remainder of the 1580s, was considered sufficiently peaceful to allow for the removal of martial law.
Fiach then became involved with the political intrigues of the Gaelic lords in Ulster. The involvement of the O'Byrne's in the escapes of Red Hugh O'Donnell and his ultimate return to Ulster is well known and Red Hugh's return to Ulster was a vital step in the forging of a Gaelic confederacy in Ulster.
In March 1594 Fiach attacked Ardree Castle, near Athy, and burned it to the ground, killing all eight occupants. The proximity of the revolt to Dublin worried the authorities. Martial law was reintroduced and the taming of the O'Byrnes made a priority. Russell launched a major offensive against Fiach in January 1595 and succeeded in driving him from his Ballinacor base. Having achieved this, he proceeded to establish a military garrison at Glenmalure, a Gaelic stronghold for the previous three hundred years. The capture of the Ballinacor base signalled the beginning of the end of the O'Byrne dominance in the uplands. Garrisons at Newcastle, Castlekevin, Enniscorthy and now Glenmalure left McHugh with little room to manoeuvre and over the succeeding months Russell proceeded to attack and weaken McHugh's army.
In 1595, Fiach attacked Ballinacor in an effort to recover his former possession - a government garrison was as much an embarrassment for Fiach as it was a threat to him - and successfully captured it. Russell was dispatched to the uplands to again sortie with Fiach. Fiach's only hope of success depended on the successful arrival of an armada from Spain and its destruction in October sealed his fate. Rebel troops began to disband following the loss of Spanish aid. In May 1597 troops tracked Fiach to a cave near Glenmalure where he was killed. Fiach's death signalled the beginning of the end of rebellion in Wicklow.