linguistic:voice_change
Irish campaign: 1649–50

Cromwell led a Parliamentary invasion of Ireland from 1649 to 1650. Parliament's key opposition was the military threat posed by the alliance of the Irish Confederate Catholics and English royalists (signed in 1649). The Confederate-Royalist alliance was judged to be the biggest single threat facing the Commonwealth. However, the political situation in Ireland in 1649 was extremely fractured: there were also separate forces of Irish Catholics who were opposed to the Royalist alliance, and Protestant Royalist forces that were gradually moving towards Parliament. Cromwell said in a speech to the army Council on 23 March that "I had rather be overthrown by a Cavalierish interest than a Scotch interest; I had rather be overthrown by a Scotch interest than an Irish interest and I think of all this is the most dangerous".

Cromwell's hostility to the Irish was religious as well as political. He was passionately opposed to the Catholic Church, which he saw as denying the primacy of the Bible in favour of papal and clerical authority, and which he blamed for suspected tyranny and persecution of Protestants in continental Europe. Cromwell's association of Catholicism with persecution was deepened with the Irish Rebellion of 1641. This rebellion, although intended to be bloodless, was marked by massacres of English and Scottish Protestant settlers by Irish ("Gaels") and Old English in Ireland, and Highland Scot Catholics in Ireland. These settlers had settled on land seized from former, native Catholic owners to make way for the non-native Protestants. These factors contributed to the brutality of the Cromwell military campaign in Ireland.

Parliament had planned to re-conquer Ireland since 1641 and had already sent an invasion force there in 1647. Cromwell's invasion of 1649 was much larger and, with the civil war in England over, could be regularly reinforced and re-supplied. His nine-month military campaign was brief and effective, though it did not end the war in Ireland. Before his invasion, Parliamentarian forces held outposts only in Dublin and Derry. When he departed Ireland, they occupied most of the eastern and northern parts of the country. After he landed at Dublin on 15 August 1649 (itself only recently defended from an Irish and English Royalist attack at the Battle of Rathmines), Cromwell took the fortified port towns of Drogheda and Wexford to secure logistical supply from England. At the Siege of Drogheda in September 1649, his troops killed nearly 3,500 people after the town's capture—around 2,700 Royalist soldiers and all the men in the town carrying arms, including some civilians, prisoners and Roman Catholic priests. Cromwell wrote afterwards:

I am persuaded that this is a righteous judgment of God upon these barbarous wretches, who have imbrued their hands in so much innocent blood and that it will tend to prevent the effusion of blood for the future, which are satisfactory grounds for such actions, which otherwise cannot but work remorse and regret.

At the Siege of Wexford in October, another massacre took place under confused circumstances. While Cromwell was apparently trying to negotiate surrender terms, some of his soldiers broke into the town, killed 2,000 Irish troops and up to 1,500 civilians, and burned much of the town.

After taking Drogheda, Cromwell sent a column north to Ulster to secure the north of the country and went on to besiege Waterford, Kilkenny and Clonmel in Ireland's south-east. Kilkenny put up a fierce defence but was eventually forced to surrender on terms, as did many other towns like New Ross and Carlow, but Cromwell failed to take Waterford, and at the siege of Clonmel in May 1650 he lost up to 2,000 men in abortive assaults before the town surrendered.

One of Cromwell's major victories in Ireland was diplomatic rather than military. With the help of Roger Boyle, 1st Earl of Orrery, he persuaded the Protestant Royalist troops in Cork to change sides and fight with the Parliament. At this point, word reached Cromwell that Charles II (son of Charles I) had landed in Scotland from exile in France and been proclaimed King by the Covenanter regime. Cromwell therefore returned to England from Youghal on 26 May 1650 to counter this threat.

The Parliamentarian conquest of Ireland dragged on for almost three years after Cromwell's departure. The campaigns under Cromwell's successors Henry Ireton and Edmund Ludlow consisted mostly of long sieges of fortified cities and guerrilla warfare in the countryside, with English troops suffering from attacks by Irish toráidhe (guerilla fighters). The last Catholic-held town, Galway, surrendered in April 1652 and the last Irish Catholic troops capitulated in April 1653 in County Cavan.

In the wake of the Commonwealth's conquest of the island of Ireland, public practice of Roman Catholicism was banned and Catholic priests were killed when captured. All Catholic-owned land was confiscated under the Act for the Settlement of Ireland of 1652 and given to Scottish and English settlers, Parliament's financial creditors and Parliamentary soldiers. Remaining Catholic landowners were allocated poorer land in the province of Connacht.