Between 1639 and 1651 a war took place in the English islands that history calls the War of the Three Kingdoms. It was about a series of battles and conflicts that took place in England, Ireland and Scotland shortly after all three territories came under the same monarch. Among these conflicts the best known is undoubtedly the English Civil War and the conflicts focused more than anything on tensions between the king and social and religious issues. Was the king a representative of God on earth or not? Could their actions be controlled by a parliament? And in that situation both Ireland and Scotland rebelled against the English.
Finally in England the forces of Parliament, led by Oliver Cromwell. In Ireland in 1641 the so-called Irish Revolution took place and in 1649 the Cromwel's conquest of Ireland. That is, the conquest of rebellious Irish territory by the English parliamentary forces. By then, since the Irish rebellion, Ireland was controlled by a Catholic Confederation that had a signed alliance with the English royalists, the losers of the English Civil War, it is worth remembering. But well, finally Cromwell's forces defeated the Irish and occupied the country.
Some penal laws were then passed against the Catholics, who were the vast majority of the country, and vast amounts of land were confiscated and generally distributed to the military who had participated in the conflicts. Cromwell's forces in Ireland were truly brutal and that is why this character is still quite hated. According to some historians the slaughter was such that the total population of Ireland fell between 15 and 25%, although some say that half of the population was eliminated.