Both battalions proceeded to Ireland in 1800, the first under the command of Lieut.-Colonel Madden, and the second under Major Lord Sinclair; Lieut.-Colonel Barry afterwards succeeded to the command of the first battalion.
1802
In 1802, the war was terminated by the treaty of Amiens; when the British army was reduced, and the second battalion was disbanded,—the establishment of the regiment being fixed at seven hundred and fifty rank and file.
1803
The conduct of Napoleon Buonaparte, then First Consul of France, occasioned hostilities to be resumed in 1803, when the British army was augmented, and preparations made to repel a threatened invasion of the French. The Fifteenth regiment remained in Ireland.
1804
Preparations to repel the menaced French invasion were continued in 1804, and a second battalion was added to the regiment. It was formed of men raised for limited service under the Additional Force Act passed on the 29th of June, 1804, and was quartered at Scarborough, in Yorkshire.
1805
During the winter, the first battalion again embarked for the West Indies, and landed at Barbadoes on the 12th of March, 1805. In May, Surgeon Shaw died at Barbadoes.
This year is celebrated in the naval annals of Great Britain for the splendid achievements of the marine forces of the kingdom. Rear-Admiral Viscount Nelson having proceeded to the West Indies in quest of the French fleet, the Fifteenth regiment received orders to embark and serve as marines. It went on board on the 4th of June, Lieut.-Colonel Barry and the staff being appointed to the "Conqueror" of seventy-four guns; but after a cruise of fourteen days, the fleet returned to port, and the regiment landed: it therefore missed sharing in the glorious victory of Trafalgar, on the 21st of October, 1805. During the months of July, August, and September, the regiment lost nine officers and two hundred and twelve men by fever.