Sligo Town was an excellent advance base, with Ballyshannon 20 miles to the north-east commanding an important river-ford at the principal western passage into O'Donnell's country in Ulster.

He then marched to Dunavaragh with 1,500 of his men, where he was joined by the additional forces of Brian Óg, King of West Breifne, who had 400 soldiers stationed at the pass, as well as those of Conor MacDermott.

But Clifford had received false intelligence that the pass was undefended, and he therefore chose to seize the opportunity and march across, promising his troops plenty of beef in the evening.

The barricade was lightly defended by just a handful of Irish troops who quickly abandoned it and rode on horseback to their army's camp to inform Red Hugh O'Donnell of the advancing English forces.

Immediately following the news of the advancing English force, Conor MacDermott and his 600 troops moved swiftly into the mountains and took up concealed positions on either side of the bog road near the Bohar-Buidhe Woods.

However, MacDermott noticing the heavy casualties the initial fusillades had caused in the English ranks, ordered his pipers to continue sounding battle songs and believed he could destroy Clifford's entire force.

At this point, Brian Óg and his 160 heavy Gallowglasses entered the battle, causing panic amongst the English forces:Like hounds slipped from the leash, O'Rourke's Brenny men went upon the Queen's vanguard... MacDermott's gunmen and archers gave way to the right hand and to the left, opening out like folding doors as the Brenny men, with a shout at such an instant changed fortitude to alarm, and alarm to panic terror, went upon the foe.

With the English ranks in disarray, the main body of Irish infantry, which had concealed itself on the reverse slope of the hill, closed in and fought hand-to-hand.

As Markham's forces pursued MacDermott's fighters – and English troops fired from the roadside at the gunmen on either side of the bog, Brian Óg led a charge of his own.

[4] Though the actions of the English cavalry allowed many of their foot soldiers to escape, Clifford's men were pursued as far as the town of Boyle by 400 of MacDermott's musketeers and gunmen alongside the 160 gallowglasses of Brian ÓG.

Brian Óg O'Rourke, who had led the soldiers on the ground, ordered Clifford's head to be cut off and delivered to O'Donnell, who had remained nearby, but without taking part in the fight.

[5] While the head was brought to Collooney Castle to intimidate its defenders, the trunk was carried by MacDermott to the monastery of Lough Key, where he hoped to use it to ransom his own prisoners.

But Clifford had been overconfident – a trait in him that Essex once warned against – and it is clear that English military commanders were choosing to learn the hard way about the increased effectiveness of Irish rebel forces.

There is an impressionistic sculpture by Maurice Harron called "The Gaelic Chieftain", unveiled in 1999, overlooking the N4, but this is not on the site of the battle, which is roughly 2 kilometres south-west of the statue.