Battle of Knockdoe The Battle of Knockdoe was a conflict between the Hiberno - Norman de Burghs ( Burkes ) and Anglo - Norman Fitzgeralds , along with their respective Irish allies . On 19 August 1504 , the Parish of Lackagh ( Irish " Leacach " ) was the site of what appears to have been an unusually bloody conflict , arising from a dispute between Maelsechlainn mac Tadhg Ó Cellaigh ( Mod . Irish " Maoilseachlainn mac Thaidhg Uí Cheallaigh " ) , King of Ui Maine – Mod . Irish " Uí Mháine " ) and Ulick Fionn Burke , the Clanricarde . Background . Ulick Finn , as Burke was called , was an aggressive local magnate . He had become The Clanrickarde in the year 1485 , and sought to establish his authority over all Connacht , including County Mayo , where the other branch of the great De Burgo ( Burke ) family held power . Although both families were of Norman stock , the western de Burghs ( or Burkes ) were integrated into the Gaelic world , whereas the Fitzgeralds of the Pale , though Gaelicised , retained cultural , social and political links to England . The King ' s Deputy , Gerald , Earl of KIldare ( Gearóid Mór ) , became concerned that Ulick Burke ' s attempt at supremacy in Connacht could threaten his claim to be the paramount magnate in Ireland . He tried to persuade Ulick to acknowledge his authority by giving him his daughter Estacia in marriage . But Ulick Burke resisted all attempts to have his tenantry and power absorbed by the Earl of Kildare , forming an alliance with O ' Brien of Thomond and the magnates of Munster . The Burkes of Mayo , on the other hand , joined forces with Kildare with a view to suppressing their dangerous neighbour . Personal reasons . In 1503 Ulick Burke attacked and destroyed the castles of O ' Kelly , Lord of Hymany , at Monivea ( " Muine Mheá " ) , Garbally ( " Gallach " ) and Castleblakeney ( " Garbhdhoire " ) . The Irish sources attest that O ' Kelly complained of this to the Lord Deputy . Burke appears to have also taken up with O ' Kelly ' s wife , and there may have been ill - feeling between the Lord Deputy and Burke because of the latter ' s treatment of Gearóid Mór ' s daughter . The battle . It appears that for political ( and possibly for personal ) reasons the Lord Deputy was eager to help O ' Kelly weaken the prestige of Clanrickarde . Both sides gathered to their side a large contingent of lesser magnates and their armies . The Lord Deputy ' s forces included contingents from Leinster , Ulster and Connacht , among which were the armies of Red Hugh O ' Donnell ( Aodh Ruadh Ó Domhnaill ) and Art Ó Néill , the McDermotts and Morrisroes of Connacht and a contingent provided by O ' Kelly . Facing them were the forces of Burke and his allies – the O ' Briens of Thomond , the McNamaras , the O ' Kennedys and the O ' Carrolls . The armies met on the slopes of Knockdoe , almost a mile to the north of Lackagh Parish Church , with heavily armed Gallowglass playing a large part on both sides . The battle appears to have lasted all day , with the heaviest fighting ( according to tradition ) taking place along the River Clare in the townland of Ballybrone ( " Baile Bhróin " ) . The precise number of casualties is unknown , though contemporary observers , as evidenced in later chronicles , were impressed by the extent of the slaughter . Round the summit of Knockdoe are many cairns ( burial mounds ) where , by tradition , the dead are said to have been buried , with one in particular being pointed out as the resting place of the two sons of O ' Brien of Thomond . The Lord Deputy , though victorious , had many among the slain . His army remained the night on the field as a token of victory , then marched to Galway , looting Claregalway castle en route and taking as prisoners the two sons and daughter of Ulick Burke . They remained in Galway for a few days and departed to Athenry , which they captured . The Clanrickarde Burkes faded into obscurity for some decades , with their rivals , the Mayo Burkes , gaining influence as a consequence . It is said that firearms were employed in the course of the battle , an early instance of their use in Ireland .