The Siege of Clonmel took place during the Cromwellian conquest of Ireland when the town of Clonmel in southern Ireland was besieged by Oliver Cromwell’s New Model Army. Cromwell's 8000 men eventually took the town from its 1000 irish defenders, but not before they suffered heavy losses.

The town was defended by Hugh Dubh ("Black Hugh") O’Neill, a veteran of siege warfare in the Thirty Years War and experienced soldiers from the Irish Ulster army. Cromwell was in a hurry to take the town as he was being summoned back to England by the English Parliament to deal with a Royalist uprising there. As a result he tried to take Clonmel imediately by assault, rather than opt for a lengthy siege.Cromwell’s artillery battered a breach in the town walls, which his infantry was supposed to storm and then open the main town gate to let in Cromwell himself and the Parliamentarian cavalry.

However, O’Neill put all able bodied townspeople to work building a parapet inside the breach lined with artillery, muskets and pikemen. The parapet was v-shaped, starting at the mouth of the breach and narrowing until it ended about 50 metres inside the town. At the end of the breach, O'Neill positioned two cannon, loaded with chain shot. The breach, as a result was, in military terms, a "killing ground". The Parliamentarian infantry who assaulted the breach were repeatedly cut down by the concentrated musket and cannon fire, until the soldiers finally refused to make any further attacks on what appeared to be a death trap. Cromwell then appealed to his elite cavalry, the Ironsides to make a fresh assault on foot. They assaulted the breach for three hours, taking heavy casualties, but failing to break into the town. Eventually, as night fell, Cromwell called off the assault.

However, O’Neill’s men were out of ammunition and slipped away under cover of darkness - making their way to Waterford. Cromwell negotiated a surrender with the town’s mayor, believing that Clonmel was still heavily defended. The surrender terms stipulated that the lives and property of the townspeople would be respected. Cromwell, although angry at the deception, did not allow his soldiers to abuse the terms of the surrender when he found that the garrison was gone and the town defenceless. His admirers cite this as an example of his integrity while his critics contrast it with the massacre he ordered at Drogheda the previous year.

The New Model Army lost at least 1500 men killed at Clonmel, and possibly as many as 2500, with hundreds more wounded, its largest ever loss in a single day. However it successfully completed the conquest of Ireland in the next two years.

See also

• Irish Confederate Wars
• Wars of the Three Kingdoms
• Irish battles