After holding administrative posts in London, he was despatched to the Kingdom of Ireland in June 1798 after the Irish Rebellion broke out. In addition to mopping up most of the remnants of the rebellion, Cornwallis was instrumental in convincing the Irish Parliament to pass the Act of Union (1800). This was a critical step in uniting the British and Irish crowns, creating the United Kingdom of Great Britain and Ireland. Cornwallis resigned his posts in Ireland in 1801 after King George refused to support Catholic emancipation.