The cannon ball dates from the siege of Waterford by the army of Oliver Cromwell in 1650 when ships on the river Suir bombarded the city with cannon fire.
A similar cannonball is on display on the top floor of the tower.
Waterford was the only city that Oliver Cromwell - the most successful general in Europe in the 17th century - failed to capture.
In the troubled 1640s when England was torn apart by civil war between the army of King Charles I and and that of his parliament led by Oliver Cromwell, Waterford city was under Catholic control. Following the defeat and execution of the king Oliver Cromwell turned his attention to Ireland and landed here in 1649. By November he was outside the walls of Waterford and demended the surrender of the city. The garrison in Waterford refused and Cromwell laid siege to the city. However he failed to capture Waterford and by December he was forced to break off the siege and move his army to winter quarters. Soon afterwards Cromwell left Ireland and returned to England.
The respite for Waterford was short lived however. The following June his army was back - this time led by his son-in-law General Henry Ireton. By now the city garrison was short of food and munitions. General Ireton commenced an artillery bombardment of the city. His ships sailed up the river and bombarded the walls - including Reginald's Tower - with cannon. The cannonball lodged in the wall of the tower dates from this siege.
The defending garrison were only able to offer a token resistance and within a short period Waterford fell to his troops. They ransacked the city in serch of valuables and paid particular attention to the churches - looking for the gold and silver chalices and other church plate.
However the city's greatest treasure escaped destruction - the magnificent set of cloth-of-gold medieval vestments which are now on display in the Medieval Museum.