The Cromwellian conquest of Ireland or Cromwellian war in Ireland (1649–1653) was the re-conquest of Ireland by the forces of the English Parliament, led by Oliver Cromwell, during the Wars of the Three Kingdoms. Cromwell invaded Ireland with the New Model Army on behalf of England's Rump Parliament in August 1649.

Following the Irish Rebellion of 1641, most of Ireland came 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, ending 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. As punishment for the rebellion of 1641, almost all lands owned by Irish Catholics were confiscated and given to British settlers. The remaining Catholic landowners were transplanted to Connacht. The Act of Settlement 1652 formalised the change in land ownership. Catholics were barred from the Irish Parliament altogether, forbidden to live in towns and from marrying Protestants.

The Parliamentarian conquest was brutal, and Cromwell remains a deeply reviled figure in Ireland. The extent to which Cromwell, who was in direct command for the first year of the campaign, was responsible for the atrocities is debated to this day. Some historians have argued that the actions of Cromwell were within what many empires at the time viewed as accepted rules of war, while others disagree.

The impact of the war on the Irish population was unquestionably severe and although there is no consensus as to the magnitude of the loss of life, most modern estimates generally fall in between 15 and 50% of the native population. The war resulted in famine, which was worsened by an outbreak of bubonic plague. Older estimates of the drop in the Irish population resulting from the Parliamentarian campaign reach as high as 83 percent. The Parliamentarians also transported about 50,000 people as indentured labourers to the English colonies in North America and Caribbean. Some estimates cover population losses over the course of the Conquest Period (1649–52) only, while others cover the period of the Conquest to 1653 and the period of the Cromwellian Settlement from August 1652 to 1659 together.