 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 . 
