The mills were founded in 1794 by Corkman Charles Henry Leslie and his silent partner John Travers. Leslie chose the flat valley of Ballincollig for practical reasons: it was remote enough for safety, close enough to Cork's port for importing raw materials, and the River Lee could power his waterwheels via a purpose-built canal one and a half miles long. For two decades, the Ballincollig mills were the largest in Ireland and second largest in Europe, behind only Waltham Abbey in England. When the 1798 Rebellion and Napoleon's expanding ambitions made gunpowder a strategic priority, the British Board of Ordnance bought the mills on a 999-year lease for 30,000 Irish pounds. The site expanded tenfold -- twelve new mills, processing buildings, workers' homes, and a barracks enclosed within a high limestone wall across 435 acres.

Making gunpowder was lethal work, and the Ballincollig mills claimed lives with grim regularity. An 1859 explosion in the dusting house killed five men -- Timothy Burns, John Corcoran, William Looney, Timothy Lyons, and James Merrick. A contemporary account described the scene with haunting precision: relatives gathered in "loud and incessant lamentations" while others arrived carrying "wrapped in grass or cloth, a blackened cinder that was once a hand, a foot, or other portions of one of the dead men." Body parts were found on the hillside half a mile away. Two years later, another explosion in the pressing mill killed five more. Between 1841 and 1877, at least a dozen major explosions rocked the complex, each one adding names to the toll.

Gunpowder demanded specialized trades. Coopers fashioned oak barrels bound with copper hoops, fitted tight enough to keep moisture from the powder. By the late 1830s, they were producing 16,000 barrels a year. Apprentices served five years, beginning at age fourteen, governed by the rules of the Cork Coopers Society. Fathers passed the trade to sons. But from the 1870s, metal canisters began replacing wooden barrels, and imported casks from the Royal Arsenal at Woolwich undercut local production. The coopers' wages fell by a third. Families emigrated to England. When the mills finally closed in June 1903, displaced coopers begged the Cork Coopers Society for permission to seek work in the city. The Society refused.

While famine devastated other parts of Ireland in the 1840s, Ballincollig grew. At its peak, some 500 men and boys worked the mills, and the village's population reached 1,130. By mid-century, most of the powder was blasting charge for railway construction, mining, and quarrying rather than military ordnance. But the end of the Boer War in 1903 killed demand for good, and the mills closed permanently. The site passed to Imperial Chemical Industries, then in 1974 to Cork County Council, which developed it into Ballincollig Regional Park. A Heritage Centre opened in 1993 with a reconstructed incorporating mill, but closed on financial grounds in 2002. In 2022, the restored mill was burned down by vandals -- a final, pointless destruction in a place that had known so much of it.