If you look carefully at the stone wall surrounding the castle, you may notice an iron cannonball embedded near the top of the wall. According to tradition, the ball is a legacy of the 1599 siege of the castle, when it was attacked by Crown forces under the command of Robert Deveraux, Earl of Essex. He was once a favourite of Queen Elizabeth Ist, and he had been sent to act as Lord Lieutenant of Ireland in 1599, during the conflict that became known as the Nine Years War. He was well supported with one of the most powerful armies ever deployed in Ireland, of around 19,000 foot soldiers and 1300 horse. Despite the investment in his army, when Essex arrived he was faced with a difficult situation, after the Battle of the Yellow Ford (1598), the Irish forces under the fine strategist Earl of Tyrone, had nearly broken the English hold on Ireland. Essex had a growing reputation after success in Spain, but he viewed the Irish conflict disparagingly, as being unworthy of his attention. He first relieved the siege on Portlaoise, then secured the pass of Cashel and then continued to Cahir Castle.
Essex marched on Cahir with approximately 3,000 foot soldiers, 200 calvary, cannons and culverin. These artillery pieces were dragged by manpower all the way from Clonmel to Cahir, which would have been exhausting work The siege on the castle lasted for three days before the artillery breached the walls. James Gillada Butler, who led the defence of the castle in opposition to his older brother the Earl of Ormonde, managed to escape by jumping into the swirling waters of the River Suir and swimming away. Essex was thrilled with his victory, however it was of little strategic value for the campaign. Unwilling to come to grips with Hugh O’Neill, Essex floundered about the country, ineffectually before deciding to take matters into his own hands. Against orders he followed his own initiative and decided to negotiate with Hugh O’Neill. This did not go down well in the English Court.