The Nine Years' War was nearly over by the time it reached Dunboy. The Gaelic alliance that had risen against Elizabethan rule had pinned its last hopes on a Spanish landing at Kinsale in 1601 - a force commanded by Don Juan del Aguila and sent by King Philip III of Spain. The landing failed. By January 1602, Aguila had surrendered to the English Lord Deputy, Lord Mountjoy. Most of the Gaelic lords sought terms. O'Sullivan Beare did not. He took back Dunboy from the small Spanish garrison still holding it, kept the arms and powder they had stored there, and resolved to fight on. The castle had been built in the fifteenth century to control the harbour of Bearhaven. Now it would be the last redoubt of a war the Gaelic Irish had already lost.

The defenders were not a token garrison. O'Sullivan left his best men - one hundred and forty-three of them - under MacGeoghegan's command, with Friar Dominic Collins to give them the sacraments. The castle was thought impregnable. The walls were thick, the position commanded the harbour, and the men inside had Spanish ordnance and Spanish powder. What they did not have was time, or numbers, or any reasonable hope. Carew arrived with somewhere between four and five thousand soldiers, supported by warships that could shell the castle from the sea. A defender's cousin, Owen O'Sullivan of Carrignass, had defected to the English and told Carew where the wall was weakest: a stairwell that ran up the southern face. The English guns concentrated their fire there. The defenders watched their own walls dissolve into rubble over ten days of bombardment.

By the tenth day the castle was ruins. MacGeoghegan, whose son Bryan had already been killed in the fighting, sent a messenger to Carew to request terms. Under the rules of war as the English understood them, asking for terms after the battle had begun forfeited the right to surrender. Carew hanged the messenger in sight of the walls. Some defenders swam for nearby Bere Island and were cut down or captured in the water. The rest fell back into the castle's cellar and waited for the assault. It came on the eleventh day. The fighting was hand to hand among the broken stones. MacGeoghegan, mortally wounded, dragged himself toward the gunpowder stores to ignite them and take the cellar down with the attackers - Captain Power of the English force hacked him down before he reached the kegs. The defenders who survived the final assault were marched to the market square at nearby Castletown Berehaven and hanged.

Three captives were kept alive for interrogation. Two were eventually hanged when they would not give the information their captors wanted. The third was Dominic Collins, the Jesuit friar from Youghal who had ministered to the garrison through the siege. Carew offered him his life if he would take the oath of supremacy and acknowledge Elizabeth I as head of the Church. Collins refused. He was taken back to his home town and hanged there in October 1602 - a deliberate choice meant to make an example of him among the people who had known him as a child. Three centuries later, in 1992, Pope John Paul II beatified him. On the wall of the ruined castle today, a plaque in Irish and English remembers the heroes who fell at Dunboy in June 1602 for country and faith. A separate stone marks the Dursey Island massacre of the same campaign, in which Carew's men killed roughly three hundred O'Sullivan retainers, women and children among them, after taking the fort there.

O'Sullivan Beare, hearing of Dunboy's fall, fought a guerrilla war through the southwest for six months before the weight of English forces and the coming winter made his position impossible. On the last day of 1602, he set out with about a thousand of his people - fighters, women, children, the old - to walk north to friendly territory in West Breifne, more than three hundred miles across hostile country in deep winter. They were attacked at river crossings, harried in the mountains, and worn down by hunger and cold. When the survivors reached O'Rourke's castle in early 1603, thirty-five were left. Others had been lost along the way, or had stopped where they could and stayed; their descendants in the midlands are still called 'the Beres'. O'Sullivan eventually died in exile in Spain, murdered in a Madrid street in 1618. The castle he could not save lies in ruins above the harbour, the grass growing through its broken walls.