An English army of up to 5,000 under Sir George Carew besieged the castle, which was held by a Gaelic Irish force of 143 loyal to Donal Cam O'Sullivan Beare.

It was a stone tower house, built to control and defend the harbour of Bearhaven, which was a stronghold of Donal Cam O'Sullivan Beare, a Gaelic leader and the 'Chief of Dunboy'.

He was helped by King Philip III of Spain, who sent an invasion force to Kinsale under the command of Don Juan del Águila.

[8] In February, as part of the terms of Águila's surrender to Mountjoy, Saavedra was preparing to hand the castle over to English forces when he and his men were overpowered and disarmed by O'Sullivans (who later released them for transportation back to Spain).

He left a force of 143 of his best men to defend the castle under the charge of Captain Richard MacGeoghegan and care of Friar Dominic Collins.

But before the siege got under way, O'Sullivan himself and most of his army had already marched to another of his strongholds, Ardea Castle, on the northern coast of the Beara peninsula, to secure money and supplies that had just arrived by ship from Spain.

[13][14][10] During the siege of Dunboy, an English detachment under Carew's command also besieged a fort held by the O'Sullivans on nearby Dursey Island.

[4] According to Philip O'Sullivan Beare, all 300 of the fort's occupants, including women and children who were sheltering there, were killed by Carew's men under his orders in what became known as the Dursey Island massacre.