His terror methods in Ireland, which were described as “playing the very Devil with the country,” are directly credited with fanning the flames of the 1798 Rebellion throughout the country.
Lake succeeded Sir Ralph Abercromby in command of the troops in April 1798, issued a proclamation ordering the surrender of all arms by the civil population of Ulster unleashing a reign of terror which became known as the "dragooning of Ulster".
Lake then took overall command of a force of some 20,000 troops to crush the successful Wexford rebels and defeated the main rebel army at Vinegar Hill (near Enniscorthy, County Wexford) on 21 June. His policy of brutality towards rebels found in arms brought him into conflict with the more humane Lord Cornwallis who had now assumed the chief command in Ireland and had instituted an Amnesty Act to encourage rebels to lay down their arms.
In August Cornwallis sent Lake to oppose a French expedition of 1,000 troops which had landed at Killala Bay, County Mayo on 23rd August. On the 27th of the same month Lake arrived at Castlebar with a force of 6,000 troops, but only to witness the humiliating rout of his troops,(The Races at Castlebar). He eventually retrieved this disaster by forcing the surrender of the French at the Battle of Ballinamuck on 8 September.