The Battle of Curlew Pass is fought on August 15, 1599, during the campaign of the Earl of Essex in the Nine Years’ War, between an English force under Sir Conyers Clifford and a rebel Irish force led by Hugh Roe O’Donnell. The English are ambushed and routed while marching through a pass in the Curlew Mountains, near the town of Boyle, in northwestern Ireland. The English forces suffer heavy casualties. Losses by allied Irish forces are not recorded but are probably minimal.

In April 1599, Robert Devereux, 2nd Earl of Essex, lands in Ireland with over 17,000 troops and cavalry to put down the rebellion of Hugh O’Neill and Red Hugh O’Donnell, which has spread from Ulster to all Ireland. To this end, he supports an Irish enemy of O’Donnell’s, Sir Donogh O’Connor (O’Connor Sligo), encouraging him to repossess those territories of his in Sligo that O’Donnell has occupied. Sligo Town is an excellent advance base, with Ballyshannon 20 miles to the northeast commanding an important river-ford at the principal western passage into O’Donnell’s country in Ulster. English military advisers have long urged the government councils in Dublin and London to capture these strategic points. O’Connor’s brother-in-law, Tibbot ne Long Bourke, is appointed joint-commander with an English captain of a force sailing from Galway, and O’Connor is expected to receive them in Sligo. However, O’Donnell quickly besieges O’Connor at Collooney Castle with over 2,000 men in an effort to starve him out, and Essex is put on the back foot. Essex has no option but to support the besieged O’Connor, one of the few Gaelic chieftains the Crown can rely upon for support. He orders the experienced Sir Conyers Clifford, who is based in Athlone, to relieve the castle with 1,500 English infantry and 200 cavalry. It is hoped that the operation would also distract the chief rebel, O’Neill, and afford the crown an opportunity to march into his Ulster territory across its southeastern border.

O’Donnell leaves 300 men at Collooney Castle under his cousin, Niall Garbh O’Donnell, and sends another 600 to Sligo town to prevent the landing of English reinforcements under Tibbot ne Long Bourke. He then marches to Dunavaragh with 1,500 of his men, where he is joined by additional forces under local chieftains Conor MacDermott and Brian Óg na Samhthach Ó Ruairc. The Irish then carefully prepare an ambush site in the Curlew Mountains, along the English line of march. O’Donnell has trees felled and placed along the road to impede their progress. When he gets word of the English passing through Boyle, O’Donnell positions his men. Musketeers, archers and javelin men are placed in the woods alongside the road to harass the English. The main body of Irish infantry, armed with pikes and axes, are placed out of sight behind the ridge of the mountain.

In hot harvest weather, Clifford’s force marches from Athlone through Roscommon, Tulsk and Boyle. At 4:00 PM on August 15, they reach the foot of the Curlew Mountains, which have to be crossed before Sligo can be approached. The expedition is poorly supplied, and Clifford’s men are tired and hungry, and probably in no fit state to continue. But Clifford has received false intelligence that the pass is undefended, and he therefore chooses to seize the opportunity and march across, promising his troops plenty of beef in the evening. This means that his men miss out on the rest that had been planned for them in Boyle, whereas the Irish are well fed and prepared.

The English come under gunfire, arrow and javelin attack as soon as they reach the first of O’Donnell’s barricades, between Boyle and Ballinafad. The barricade is immediately abandoned by the Irish but as the English moved past and proceed up the hill they sustain further casualties. The road consists of “stones of six or seven foot broad, lying above ground, with plashes of bog between them,” and is lined with woodland on one side. The further the English advance, the more intensive the rebels’ fire becomes, and some English soldiers begin to lose their nerve and slip away. Eventually, there is a firefight, lasting about 90 minutes, at the end of which the English vanguard has run out of gunpowder. The commander of the vanguard, Alexander Radcliffe, can no longer control his troops. They wheel about in a panic and collide with the main column, which breaks and flees. The commander leads a charge with his remaining pikemen but is shot dead. With the English ranks in disarray, the main body of Irish infantry, which has concealed itself on the reverse slope of the hill, closes in and fights hand to hand. Clifford tries to regain control over his men, but appears overcome by his circumstances. He manages to rally himself and is killed by a pike-thrust as he rushes the enemy. The English are routed, but the situation is prevented from becoming a complete disaster for them when the commander of the horse, Sir Griffin Markham, charges uphill and temporarily drives the rebels back.

Though the actions of the English cavalry allows many of their foot soldiers to escape, Clifford’s men are pursued as far as the town of Boyle, where they find shelter in Boyle Abbey. About 500 English are killed in the battle. Irish losses are not recorded, but are probably small, having been firing from prepared positions and then routing a disorganised and demoralised enemy.

Clifford’s head is cut off and delivered to O’Donnell, who has remained nearby but without taking part in the fight. While the head is brought to Collooney Castle to intimidate its defenders, the trunk is carried by MacDermott to the monastery of Lough Key, where he hopes to use it to ransom his own prisoners. At last, the trunk is given a decent burial in the monastery.

O’Connor Sligo surrenders the castle shortly afterwards and reluctantly joins with the rebels. After the victory, there is a noticeable increase in the rate of desertion by Irish troops from the ranks of Essex’s army, and the earl orders that the surviving troops be divided up as fit only to hold walls.

The battle is a classic Gaelic Irish ambush, similar to the Battle of Glenmalure in 1580 or the Battle of the Yellow Ford in 1598. According to the Annals of the Four Masters, the victory is put down to the intercession of the Blessed Mary, rather than to arms. But Clifford had been overconfident, a trait in him that Essex once warned against, and it is clear that English military commanders are choosing to learn the hard way about the increased effectiveness of Irish rebel forces. Queen Elizabeth I‘s principal secretary, Sir Robert Cecil, rates this defeat (and the simultaneous defeat of Sir Henry Harrington in the Battle of Deputy’s Pass in County Wicklow) as the two heaviest blows ever suffered by the English in Ireland, and seeks to lay the blame indirectly on Essex. It leaves O’Donnell and O’Neill free from any threat from the Connacht side, and renders a land-based attack through Armagh highly improbable, a factor that weighs with Essex as he marches northward later in the year and enters a truce with O’Neill.

In August 1602, the Curlew Pass is the scene of the last victory won by the rebels during the war, when a panicking English force is again routed and suffers significant losses. This time the rebels are led by Rory O’Donnell who commands 400 musketeers.

Today the battlefield at Curlew Pass is overlooked by an impressionistic sculpture by Maurice Harron called “The Gaelic Chieftain”, unveiled in 1999.