During the reign of Queen Elizabeth I of England, the O'Byrnes controlled territory in the Wicklow mountains south of Dublin, covering about 153,000 acres (620 km2).

The Kiltimon, Downs, Cloneroe and Newrath branches of the clan were generally loyal to the Crown, having benefited under English law by primogeniture and the system of 'surrender and regrant'.

The Ranelagh O'Byrnes were unsubmissive and were reckoned capable of fielding one hundred expert swordsmen, posing a constant threat to Tudor authority within the Pale through raids on the lowlands, which weighed in the balance of power between the Butler (Ormond) and Fitzgerald (Kildare) dynasties in Leinster.

Fiach (in Irish, the raven) assisted the escape of the imprisoned Edmund Butler, when the latter fell from a rope while climbing from the battlements of Dublin Castle.

In 1572, Fiach was charged with complicity in the murder of Robert Browne of Mulcranan, son-in-law of the seneschal of County Wexford, Sir Nicholas White.

Under the government of Sir Henry Sidney, O'Byrne gave support to his own brother-in-law, Rory Oge O'More, the pretender to the lordship of Leix, who broke out in rebellion in 1577.

The O'Byrnes continued with their cattle raids, until Fiach made his submission in early 1579 at Christ Church Cathedral, Dublin, where he gave pledges of allegiance and acknowledged the authority of the crown government.

In 1579, after succeeding his father in the leadership of the O'Byrnes, Fiach joined with James Eustace, Viscount Baltinglass – despite a history of mutual enmity between their families – during the Second Desmond Rebellion.

An offensive into the province of Munster was expected, and before he might engage on that campaign Grey had to deal with the O'Byrnes to prevent them attacking him in the rear as he marched south.

He planned to enter Glenmalure from the neighbouring Glen of Imaal and attack O'Byrne's stronghold; the enemy was expected to be flushed from its fastness, whereupon the English cavalry would ride them down in their flight.

Grey altered his course and travelled several miles south, where he was joined by Kildare, before heading east in a loop and making an arduous ascent into the mountains.

O'Byrne had concealed his men in the craggy terrain, and the English troops, conspicuous in their red and blue coats and white hose, instantly found themselves sliding along a river course.

Despite this disturbing setback, Grey was in a position to post a garrison in the locality, in the hope that this would contain O'Byrne; but the raids kept coming, even into the suburbs of Dublin.

In the campaign that ensued, O'Byrne did suffer losses and failed to dislodge the garrison, but he held out, even after the crown had asserted its command in Munster with the massacre at Smerwick of the Papal invasion force.

In the following spring, when Grey passed through Wicklow, O'Byrne showed his forces on the hills and sent sorties to cut off the straggling plate wagons.

For some years after, O'Byrne remained docile and, following the death of the Earl of Desmond in 1583, had even received into his territory his old foe, Nicholas White, upon the first visit to that place by a senior crown judge.

O'Byrne fell quiet again, but Lee insisted throughout the period 1594–96 that he was a traitor to the crown, and a new initiative against him was directed by the lord deputy, Sir William Russell.

Russell set up spring camp at Shillelagh, hunting and fishing and receiving the heads of rebels, but O'Byrne was elusive and he intrigued with Hugh O'Neill, Earl of Tyrone.

At about this time his wife, Rose O'Toole, was captured and, upon her conviction for treason by a Dublin jury, sentenced to death by burning; her life was spared, only because she had been convinced to convey information to Fiach that his son Turlough was betraying him.

In his negotiations with Russell, during a truce in the Nine Years War, Hugh O'Neill included terms for the treatment of O'Byrne, and reinforced his point with the capture of the Blackwater fort, which was said to have been in response to the lord deputy's campaign in Wicklow.

However, the rest of the O'Byrnes were not considered a threat, indeed the clan were divided, with some engaging in the hunting down of Fiach, and a new fort was built at Rathdown.

Several months later the pickled head was presented to the council secretary in London by an English adventurer, who was disappointed to find that the head-silver due on O'Byrne had already been paid in Ireland.

A family genealogy brings Fiach's descendants down to 1875, in the persons of brothers Alfred and John Byrne of Sleaty, County Laois.

This makes it apparent that simple 'Aodh Ó Broin' is meant to be Aodh Gancagh of Clonmore but the later in its full form was the name of an uncle of Feach mac Aodha and brother to his mother Sadhbh.