The Cromwellian conquest of Ireland or Cromwellian war in Ireland (1649–53) refers to the conquest of Ireland by the forces of the English Parliament, led by Oliver Cromwell, during the Wars of the Three Kingdoms. Cromwell landed in Ireland with his New Model Army on behalf of England's Rump Parliament in August 1649.
Since the Irish Rebellion of 1641, most of Ireland had been under the control of the Irish Catholic Confederation. In early 1649 the Confederates allied with the English Royalists, who had been defeated by the Parliamentarians in the English Civil War. By May 1652, Cromwell's Parliamentarian army had defeated the Confederate and Royalist coalition in Ireland and occupied the country—bringing to an end the Irish Confederate Wars (or Eleven Years' War). However, guerrilla warfare continued for a further year. Cromwell passed a series of Penal Laws against Roman Catholics (the vast majority of the population) and confiscated large amounts of their land.
The Parliamentarian reconquest of Ireland was brutal, and Cromwell is still a hated figure in Ireland. The extent to which Cromwell, who was in direct command for the first year of the campaign, is responsible for the atrocities is debated to this day. Some historians argue that the actions of Cromwell were within the then-accepted rules of war, or were exaggerated or distorted by later propagandists; these claims have been challenged by others.
The impact of the war on the Irish population was unquestionably severe, although there is no consensus as to the magnitude of the loss of life. The war resulted in famine, which was worsened by an outbreak of bubonic plague. Estimates of the drop in the Irish population resulting from the Parliamentarian campaign vary from 15–25%, to half and even as much as five-sixths. The Parliamentarians also deported about 50,000 people as indentured labourers.