The O’Flaherties of Aughnanure Castle, whose ancestors formerly possessed the barony of Clare, were hereditary enemies of the De Burghs and were constantly at war with them. The De Burghs besieged the castle of Aughnanure, which surrendered after a long siege. The O’Flaherties were forced to pay an annual tribute of corn, cattle and wool to the De Burghs and their heirs forever.
For three years O’Flaherty refused to pay and De Burgh sent his son to demand payment. The young De Burgh was, on delivering his demand, murdered in the castle, his head struck off, placed in a bag and carried by O’Flaherty’s youngest son with the instructions:
“Take this bag to Tirellan Castle, and tell the Earl of Ulster that your father, O’Flaherty of Aughnanure, sent it as his tribute to him and that it is the only chiefly the O’Flaherties will every pay to him or his descendants”
Young O’Flaherty on his fathers favourite horse, Deelish, soon passed Dangan, reached the western suburbs of Galway, and having entered the town by St. James’s Gate, passed through In-Sparra-Hier, crossed the West Bridge under the middle and inner gates, rode up through Bridge Gate Street, Market Street, Goal Street, High Middle Street, when turning to the left, he passed through Little Gate Street and out by the Abbey Gate in Blake’s tower, and soon reached the outer fortification of Tirellan. Throwing the bag containing the head of young De Burgh into the courtyard, O’Flaherty galloped again through the town. The De Burghs crossed the river to intercept him at Newcastle which they reached just as O’Flaherty was riding past on Deelish. One of the pursuers threw his spear, which missed the rider but pierced the horse’s flank. Deelish, however, pressed on until through loss of blood fell dead on Borenacranny Hill.
The De Burgh followers on reaching the top of the hill were surprised to meet a large O’Flaherty force. The Earl’s men were driven back and of those who had crossed the river few lived to return. De Burgh never afterwards felt safe in Tirellan Castle, so he erected a new castle two miles beyond Galway to the north, furnished it expensively and took up his residence there. He decided that old O’Flaherty of Augnanure would have little difficulty in attacking him in his new home as it was only necessary to break down the road that led from the Abbey Gate to Tirellan Castle in order to make a passage for his galleys. He spent only one night in his new home, which received the name of Castlegar, and removed to Portumna, which became his principal residence. Tirellan was burned to the ground by the Irish garrison during the siege of Galway in 1691.