Clonmel was twice besieged. In 1516, when it was surrounded by walls and strongly fortified, it was besieged and taken by the Earl of Kildare. It's greater claim to fame was when it was attacked by Oliver Cromwell in 1650. It was defended by Hugh Dubh O'Neill of the Northern Ui Neill.
STORM UNDER SLIEVENAMON
Hugh Dubh O'Neill defends Clonmel May,1650.
And now, behold, against Clonmel. They vainly flung their bands!
Battered and bayed but undismayed the town defiant stands.
Battered and bayed but undismayed it meets each fresh attack;
With soldiers few and faint—but true—It hurls the foemen back!
—D.A.McCarthy.Undismayed by the frightful massacres at Drogheda and Wexford, Hugh Dubh O'Neill (nephew of Owen Roe) commanding 1,600 Ulster and Tipperary troops, decided to hold Clonmel while his ammunition lasted. Opposing O'Neill was a Cromwellian force of 9,000 well-armed veterans, supported by field-guns and howitzers.
The following notes are taken from a chapter in Romantic Slievenamon which it describes as "a graphic picture of the famous Siege penned in 1685 by Capt.Mulholland, a British Officer who seemingly admired "Hugh Dubh":
Cromwell sent two or three regiments of horse and foot to block off Clonmel at a distance a month before he appeared on the scene himself. Hugh Dubh defiantly turned down Cromwell's offer for him to surrender on good terms.
Despite a valiant defence, Cromwell's canons breached a section of their defensive wall. At night-fall, O'Neill sent 200 men and officers with local guides who knew a secret pathway to where they the fell on the backs of those in a fort not fully finished, killing the sixty occupants with the loss of half-a dozen. They returned safely through another gate which was opened for them.
With this respite, O'Neill set troops with men and maids to work reparing the breach in such a way that when English troops later came through, it was like entering a box canyon, and they suffered many casualties. The Siege having gone on for five or six weeks; O'Neill, having had several losses and with others wounded and sick and with ammunition low, advised Mayor Whyte that himself and his troops would leave two hours after night fall. Mayor Whyte was to wait until he had judged that O'Neill and his men were half a dozen miles from the town; then he should seek Cromwell and look for favourable terms without telling him that O'Neill had left. He succeeded in achieving this objective. Cromwell was furious when he was told that O'Neill had left and wanted to tear up the agreement. The Mayor, with the use of diplomacy and flattery, succeeded in getting him to keep his word despite the fact that he had lost about 1,500 men, more than he lost in all the towns he stormed before and since he came to Ireland. Clonmel made military history as revealing how Irish valour was well adapted for modern warfare. After the Siege, Cromwell was recalled to England.