In February 1642, Grenville was part of the army sent to suppress the Irish Rebellion, where he gained a reputation as a competent, but ruthless leader, and was appointed governor of Trim, County Meath. As a result, he missed the early stages of the First English Civil War, which began in August 1642. In June 1643, the rebels agreed a truce with Ormond, leader of the Irish Royalists, formally confirmed in September when both sides signed a Cessation of Arms.