English forces led by Reynolds and Hewson appeared before the Castle in May, but the Parliamentarian commander Hewson led some English companies away to the Wicklow Mountains to hunt the Irish partisans known as tories. This weakened the besieging force, leaving it around two and a half thousand strong.
The Connaught forces of Clanricarde and the army of Castlehaven planned to rendezvous at Tyrellspass in Westmeath. The combined force was 2,600 strong, including 800 horse. The two leaders realised that given the reduced size of the Parliamentarian force, their united army might have a chance to come to the castles relief.
James Tuchet, the English born Earl of Castlehaven was by contrast a vigorous tactical leader. With the exception of Owen Roe O'Neill, he proved to be the only Irish Confederate leader able to win major set-piece battles during the 1640s. Dubbed the 'little lord' by some Irish Confederates, on account of his short stature, he was distrusted because of his Royalist sympathies.
John Hewson was a former shoemaker who managed to work his way up the ranks of the Parliamentarian army during the English Civil Wars of the 1640s. Known for his religious extremism, Hewson became infamous as one of the signatories of the death warrant of King Charles in 1649. Later in the same year, Hewson travelled with Cromwell's army to Ireland, playing a part in the brutal capture of Drogheda in September. Later the same month, he was appointed as governor of Dublin by Cromwell, which in part explains his decision to lead hundreds of soldiers from the castle Ticroghan to pursue the Wicklow tories. This left Reynolds alone to conduct the defence of the Siege at the time of the Irish push. John Reynolds was a young man (only 25 at the time of the battle) but he was by no means green, being a veteran of the battle of Rathmines and possessing several years of fighting experience.
On 19 June the Irish column moved into the bog. At a place known as Tocar Gearr, four miles away from the castle the Irish ran into the 2,600 English soldiers deployed in a battle line. After deploying, Castlehaven ordered those cavalry men who had remained behind to distract the enemy at the flank; immediately afterwards the Irish infantry attacked, the Irish left wing (composed of Connaught men under colonel Richard Burke) attacking the English right. Shortly after nightfall Burke's men broke through, the English falling back in an ordered retreat. On the Irish right flank things did not go so smoothly. As the English army became aware of what was happening, a sudden attack was arranged against the Leinster men, led by Thomas Dillon, who made up the Irish right flank. The Irish fell back into the woods and bog. Castlehaven tried to prevent panic taking hold in the Irish centre, but failed. The remaining Irish force was thus soon in retreat. Even so, hundreds of the Irish soldiers on the left flank were able to make it to the safety of the Castle with their packages. On the way they managed to destroy part of the English siege works and capture a cannon.
In the following few days, the reinforced and encouraged garrison sallied out against the English, killing some soldiers.
On 25 June Sir Robert Talbot and Lady Fitzgerald surrendered the castle. The terms were lenient, allowing the garrison to march out with their weapons and serve elsewhere in Ireland. Reynolds however did not allow the garrison to take the cannon in the castle with them. There were suggestions at the time that Talbot had treacherously surrendered the castle earlier than needed, as there were some weeks worth of supplies remaining, but these allegations are difficult to substantiate.