Limerick's £100 million main drainage and navigation project has provided a wealth of artefacts for its newly dedicated Jim Kemmy Museum including rare mortar bombs, used in the Williamite sieges of Limerick in 1690 and 1691.
Four such bombs, three of them largely intact, have been discovered so far, each about the size of a basketball and weighing up to 15 stone.
They had a timed fuse, made from a wooden peg, through which a slow-burning powder ran.
At least one of the cast iron, hollowed-out bombs has its gunpowder intact, providing a rare insight into 17th century warfare.
Excavation of the Abbey River, a tributary of the Shannon, by a team of archaeologists this week turned up the fourth mortar bomb, which required an Army explosives ordnance disposal unit to be called out.
Comdt Dan Harvey, of the First Southern Brigade, said the ordnance unit believes the bombs were fired by catapult rather than by cannon, because they range between nine and 13 inches in diameter and would otherwise have needed mortars of a different bore to fire them.
Their effect would have been devastating, he said, between the noise and their impact. "Fired at a three-bedroom, semi-detached, you would not see much afterwards, he said.
The city held out against the first siege, but the second, in August 1691, was more successful. The besieged Jacobite forces sued for peace and the Treaty of Limerick followed.
Mr Ed O'Donovan, director of architectural excavations at the site, with his team sifted through 3,000 cubic metres of earth during the past 10 months before a 1.8 metre sewer pipe is installed as part of the drainage scheme. They had inoculations for hepatitis A and B, diphtheria, tetanus and polio.
They have found more than 5,000 objects, ranging from prehistoric arrowheads to revolvers from the 1920s. "Rivers have this ability to act as time capsules. The various phases of Limerick's history are reflected in the artefacts," he said.