FOILED in his attempt to cut off supplies from the garrison of Armagh, O'Neill at once set about reducing Portmore by starving it out. During the siege O'Donnell, who had come to his assistance, persuaded him to try an assault on the fort. Having calculated its height scaling ladders of proper length and wide enough for five men to mount abreast were made and advanced to the fort. The besieged hastened to drive off the assailants at first with a heavy discharge of artillery and as they came closer the musketeers opened fire, which was returned by the assailants. The ladders were brought forward to the Castle, but the besieged, who had learned that these ladders were being made, had deepened the ditch which surrounded the castle so that most of the ladders did not reach to the parapet and those who got to the top of them and could not get farther on account of the shortness of the ladders fruitlessly attacked the besieged. Those ladders which did reach to the top of the fort were so few that the first to ascend were easily slain before they could be succoured by their comrades. One hundred and twenty Catholics perished, and amongst them Murrough Kavanagh, a Leinster nobleman and stout soldier who had distinguished himself in Belgian wars. The others being tired out abandoned the assault and besieged the fort in the old way.
At this time Owny O'More seeing that he was unable to withstand the enemy's forces, came to Ulster and asked O'Neill to aid Leinster. Meantime Brian O'More took on the conduct of the war in Leinster and seven times successfully encountered the English and their Anglo-Irish allies from Wexford, capturing seven standards and fourteen military drums.