The Battle of Curlew Pass was fought on 15 August 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 Aodh Ruadh Ó Domhnaill (Red Hugh O'Donnell). The English were ambushed and routed while marching through a pass in the Curlew Mountains, near the town of Boyle, in the west of Ireland. The English forces suffered heavy casualties. Losses by allied Irish forces were not recorded but were probably minimal. Campaign In April 1599, the 2nd Earl of Essex landed in Ireland with over 17,000 troops and cavalry to put down the rebellion of Hugh O'Neill, and Red Hugh O'Donnell, which had spread from Ulster to all of Ireland. To this end, he supported one of O'Donnell's Irish enemies, Sir Donogh O'Connor (O'Connor Sligo), encouraging him to retake his territories in Sligo that O'Donnell had occupied. 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. English military advisers had long urged the government councils in Dublin and London to capture these strategic points. O'Connor's brother-in-law, Tibbot na Long Burke (son of Gráinne O'Malley), was appointed joint-commander with an English captain of a force sailing from Galway, and O'Connor was expected to receive them in Sligo. However, O'Donnell quickly besieged O'Connor at Collooney Castle with over 2,000 men in an effort to starve him out, and Lord Essex was put on the back foot. Essex had no option but to support the besieged O'Connor, one of the few Gaelic chieftains the Crown could rely upon for support. He ordered the experienced Sir Conyers Clifford, who was based in Athlone, to relieve the castle with 1,500 English infantry and 200 cavalry. It was hoped that the operation would also distract the chief rebel, Tyrone, and afford the Crown an opportunity to march into his Ulster territory across its south-eastern border. O'Donnell left 300 men at Collooney Castle under his cousin, Niall Garbh O'Donnell, and sent another 600 to Sligo Town to prevent the landing of English reinforcements under Tibbot na Long. 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,