The siege of Duncannon begins on January 20, 1645, during the Irish Confederate Wars. An Irish Catholic Confederate army under Thomas Preston besieges and successfully takes the town of Duncannon in County Wexford from an English Parliamentarian garrison. The siege is the first conflict in Ireland in which mortars are utilized.

At the outbreak of the Irish Rebellion of 1641, most of south-eastern Ireland falls to the Catholic insurgents. Roughly 1,000 rebels blockade Duncannon, which is heavily fortified and contains an English garrison of about 300 men. Around 150 of the English troops are killed in forays against the Irish at nearby Redmond’s Hall, but without siege artillery, or expertise in siege warfare, the rebels are unable to take Duncannon.

Hostilities continue throughout 1642, as the Irish, now organised as the Irish Confederacy raid the town’s hinterland. As in much of Ireland, the conflict is bitter. In one incident, Laurence Esmonde, 1st Baron Esmonde, the Royalist commander, hangs 16 Irish prisoners who have been taken at nearby Ramsgrange. In response, the Irish execute 18 English prisoners whom they have been holding.

In 1643, because of his need for troops to fight in the English Civil War, Charles I signs a ceasefire with the Irish Confederates. As a result, hostilities between Duncannon and the Catholic-held surrounding area are suspended.

However, in 1644, the English garrison of Cork, under Murrough O’Brien, 1st Earl of Inchiquin, unhappy with the Royalist truce with the Irish Confederates, declares for the English Parliament, who are to remain hostile to Irish Catholic forces throughout the 1640s. Esmonde, under pressure from elements of his garrison, also changes to the side of Parliament and effectively re-declares war on the Catholic Confederates. His motives are unclear: though he is a Protestant convert, the Esmonde family are Anglo-Irish Roman Catholics, and he owes his entire advancement to the Crown.

Duncannon is a strategically important town for two reasons. Firstly, it has formidable defences. Secondly and more importantly, its guns overlook the sea route to Waterford and New Ross, two of the most important Catholic-held towns and also ports at which the Confederates receive military aid from Catholic Europe.

Needing to keep this channel open and also fearing the presence of an English garrison deep in their territory, the Confederates’ Supreme Council in Kilkenny despatches Thomas Preston, general of their Leinster Army, to take Duncannon in January 1645. Preston has at his disposal 1,300 men, four cannons and a mortar. The mortar, the first of its kind to be used in Ireland has been donated by Spain the previous year and is commanded by a French military engineer named Nicholas La Loue. La Loue had served with Preston in Flanders and is chief of engineering in the Leinster Army.

Duncannon possesses formidable defences. For one thing, it is located on a peninsula and can only be approached from the north, the other three sides jutting out into the sea. Just off the town are docked four Parliamentarian ships, which are supplying Duncannon with food and reinforcements. Secondly, it possesse two lines of fortifications, the outer line being a more modern low deep rampart protected by a dry ditch and the inner wall being a medieval curtain wall, complete with three towers. However, it has two grave weaknesses, first, it is overlooked by a hill to the north, from which an attacker can fire into the town and, secondly, the water supply is located outside the walls. 

Preston arrives at Duncannon on January 20 and proceeds to construct a ring of trenches which cut off Duncannon on its landward side. From the hill that overlooks the town to the north, his guns are able to fire on a squadron of four Parliamentarian ships that are docked off Duncannon and providing the town with supplies. The flagship, the Great Louis, is badly damaged, its mast wrecked by cannon fire, and it takes several more hits from the mortar as it tries to get away. The ship sinks in deep water, drowning its crew and 200 soldiers who are on board.

Having cut off Duncannon’s supply from the sea, Preston proceeds to dig saps closer to the walls, the ultimate aim being to bring his cannon close enough to the walls in order to blast a breach and open the way for an assault. His engineers also dig a mine underneath one of the town’s bastions. All the while, the town’s defenders are kept under a bombardment by the mortar and, as the Confederate troops get closer to the walls, by sharpshooters. On March 12, one such sniper kills the fort’s second in command, one Captain Lurcan, who is hit in the head by a bullet.

On March 16, by which time the Irish trenches are “within pistol shot of the walls,” Preston orders the mine to be exploded, opening a breach in Duncannon’s outer walls. The Irish infantry then assault the town, but are beaten off with some losses. The following day, Saint Patricks Day, Preston tries again and this time his troops succeed in taking the town’s outer, more modern walls but are stopped at Duncannon’s inner, medieval ramparts. They succeed in occupying one of the town’s towers for an hour before being beaten back. Geoffrey Baron, a Confederate politician, who keeps a diary of the siege, reports that 24 Irish soldiers are killed in the two assaults.

At this point, Preston summons Esmonde to surrender, before he has to “proceed to extremities.” This is a delicate threat, implying that if the town falls to an assault, its defenders will be put to the sword – as is customary in contemporary siege warfare. Esmonde is also advised to surrender by the Parliamentarian vice admiral, William Smith, who is anchored offshore with seven ships, but cannot break through to relieve the town. In a letter that reaches Esmonde on March 11, Smith warns him that “if the rebels take the fort by storming it, they will undoubtedly put you all to death…you should agree with thy adversary while thou art in the way.” Esmond has Smith’s letter publicly read to his troops after the assaults of March 16-17 to discourage those who favour holding out.

Alongside the risk of massacre, the English garrison is also very low on gunpowder and water. The town’s only source of fresh water, a well, is behind the Confederate siege lines.

In light of these facts, Esmonde formally surrenders Duncannon to Preston on March 18. The Confederates take possession of the town but its garrison is allowed to march away to Youghal, which is in Protestant hands. However, they have to leave behind the town’s 18 artillery pieces. Esmonde himself dies a feways after the end of the siege. Preston goes on to briefly besiege Youghal, but bad weather, a lack of supplies and squabbling with James Tuchet, 3rd Earl of Castlehaven, the Confederate Munster general, puts an end to his campaign for the winter. 

The siege is of importance in that it reopens the sea route into Waterford and eliminates a hostile English garrison in Confederate territory. Preston, who had for many years been the Spanish military governor of Leuven, is highly experienced in siege warfare and his conduct of the siege draws widespread praise. Not only does he take the town, but he does so at a relatively low cost. Sixty-seven Confederate soldiers die in the siege, of whom roughly 30 die of disease. Given that the campaign is conducted in mid-winter, in an age when disease routinely kills many more soldiers than combat, this represents a considerable logistical achievement on the part of the Irish general.

The Great Lewis, the Parliamentarian ship sunk during the siege, is rediscovered in 1999 and raised in 2004.

Duncannon is besieged again during the Cromwellian conquest of Ireland by the forces of the English Parliament, as part of the Siege of Waterford. It repels a siege by Oliver Cromwell in 1649 but surrenders after a lengthy blockade by Henry Ireton in 1650.