Ireland had been a lordship under the authority of the English Crown since the twelfth century; but by the 1500s, the area under government control had shrunk to the Pale, the area around Dublin. The rest of the country was controlled by the mini-lordships of clan and feudal leaders. King Henry VIII tried to reintegrate the territory of the country by recognising the titles of the Irish nobility and giving them legal charter to their lands in return for submission to the Crown. He also created the Kingdom of Ireland in 1541, with himself as monarch. But whenever English officials tried to control the actions of Irish lords, they were invariably met with resistance. The English spent the next 50 years trying to exert their control over the Irish population, often by exceptionally brutal means. The first major conflict this caused was the Desmond Rebellions between 1569 and 1583. In the 1590s they experienced the most significant resistance, from forces in Ulster under Hugh O'Neill and Hugh Roe O'Donnell. This war is known as the Nine Years War. After some initial successes, the rebels were pinned down defending their own territory in Ulster. Since 1591, the Irish rebels had been seeking help from Spain, and in 1601, in spite of bad weather a Spanish landing finally materialised.
Phillip sent Don Juan del Águila and Don Diego Brochero to Ireland with 6,000 men, and a significant amount of arms and ammunition. Bad weather separated the ships and nine of them, carrying the majority of veteran soldiers and gunpowder, had to turn back. The remaining 4000 men disembarked at Kinsale, just south of Cork on October 2, 1601. Another force commanded by Alonso de Ocampo managed to land at Baltimore. The Spaniards rushed to fortify the precarious fortifications to withstand the approaching English armies.
On hearing of the Spanish landing, Charles Blount, Lord Mountjoy, the assigned Lord Deputy of Ireland, weakened the garrisons around the Pale and rushed to Kinsale with as many men as he could take, where he laid siege to the town. Reinforcements were brought in through Oysterhaven, the army's complement up to 12,000, but many of these fell ill, and only about 7,500 were capable of fighting.
At the same time, Hugh O'Neill and his ally O'Donnell considered their positions, before setting out - separately from each other - with a total of 5,000 infantry and 700 cavalry, on a 300 mile winter march. The combined armies of O’Neill, O’Donnell and Tyrrell came to 6,180. This included 500 of O’Sullivan Beare’s men, and 200 of Ocampo’s Spaniards.
Reinforcements arrived from Spain, and on December 24, 1601 British date: (January 3, 1602 for the Catholic Irish and Spanish armies) moved in to position. In three columns - led by Richard Tyrell, Hugh O'Neill, and O'Donnell - they marched toward a night attack, but owing to a lack of coordination and possible arguments between the commanders, they had failed to reach their destination by dawn. The English scouts were aware of the troop movements and, after leaving a number of regiments behind to guard the camp and cover Kinsale, Mountjoy led his forces to meet the enemy at a ridge northwest of the town.
O'Neill controlled the ridge, and intended to fight for it, with support from Aguila, O'Donnell, and Tyrell on multiple sides. De Aguilla, the Spanish commander, was an experienced soldier and put up a fierce defense. His instructions were, however, to hold the town until the Irish army came down from Ulster to combine with them. When neither of his allies showed signs of movement, O'Neill ordered a retreat into the marshes, hoping to mire the English cavalry in the soft land. In the end, the Irish were overpowered by the English cavalry, who charged through O'Neill's men, and prevented a flanking maneuver by O'Donnell.
The tactics showed that the Irish Foot were poorly trained for open field fighting and the formation of the hollow square. It also showed up the English cavalry techniques using the lance, as compared with the Irish method of no stirrup and overhead spear throwing.
The Irish army left the field in some disorder while the supporting Spanish army led by Ocampo tried to hold the charge and the ensuing massacre. Most fled back to Ulster, though a few remained to continue the war with O’Sullivan Beare. The Spanish, who lost many men in the siege, gave up the town to Mountjoy, "on Terms" and were allowed to sail back to Spain, not knowing that only a few days ahead another Spanish force was sent. Outnumbered, deprived from any enforcements and provisions and under constant English bombardment the Spaniards had bravely and successfully defended the town of Kinsale against all comers for more than 3 months.
O’Donnell went to Castlehaven and took a ship to Spain. He was well received there but died a few months later, said to be by poisoning by Carew’s spy, Blake.
O’Neill returned to his native Ulster, and then decided to go to Spain, and was accompanied by many supporters and lesser chieftains. This was known as the "Flight of the Earls". Their intention was always to raise an army and oust English authority in their home province, but the territories they had left behind were soon divided up in the Plantation of Ulster, and they were never able to return. The English administration saw the ideal opportunity to seize most of the land of Ulster, and to bring in Presbyterian Lowland Scots to farm it. The English had achieved their objectives of destroying the old Gaelic order, ridding themselves of the Clan system and the more troublesome chieftains. In the longer term it created the environment for centuries of sectarian strife, including the recent conflict in Ulster.