This officer, after serving with the rank of captain in the emigrant army under the Duke of Brunswick in the campaign of 1792, entered the British army sent to the Continent on the breaking out of the war in 1793. He was appointed captain in the fourth regiment of the late Irish brigade on the 1st of October, 1794, and was placed on half-pay on the 1st of March, 1798, on the reduction of that regiment; he was appointed captain in the first West India regiment on the 21st of May, 1800, and joined shortly afterwards at St. Lucia; Captain O’Connell was appointed major of brigade to the forces at Surinam in February, 1802, and served in that colony until its restoration to the Dutch in December of that year, when he joined his regiment at St. Vincent. In May, 1803, he proceeded in command of five companies to Grenada, whence he was ordered with the whole of the regiment to Dominica in 1804. On the 1st of January, 1805, he received the brevet rank of major. He commanded the light company at Roseau, in Dominica, when an attack was made on that capital on the 22nd of February, 1805, by a French force commanded by General La Grange, and successfully resisted, during the whole day, repeated attacks made by very superior numbers of the enemy on the posts occupied by Brevet Major O’Connell, with the forty-sixth regiment, his own company of the first West India regiment, and some colonial militia. He had been appointed major of brigade to the forces at Dominica in February, 1805, and on the 23rd of May following, was appointed major of the fifth West India regiment. In September he returned to England. For his services in the defence of Dominica, Major O’Connell received the thanks of the House of Assembly in that island, and was presented by that body with a sword, value one hundred guineas; he also received a sword, value fifty pounds, and a piece of plate, value one hundred pounds, from the committee of the Patriotic Fund at Lloyd’s. On the 15th of October, 1806, Major O’Connell was appointed to the SEVENTY-THIRD regiment, in which he was promoted to the rank of Lieut.-Colonel on the 4th of May, 1809, and was appointed Lieut.-Governor of New South Wales, where he continued until April, 1814, in which month he embarked in command of the first battalion of the SEVENTY-THIRD regiment for Ceylon. In January, 1815, Lieut.-Colonel O’Connell marched in command of a division of the army under Lieut.-General Robert Brownrigg into the territories of the King of Candy, the conquest of which was achieved in forty days, and crowned by the capture of the reigning monarch, who was deposed by his own subjects, and brought a prisoner to Colombo. On the 12th of August, 1819, Lieut.-Colonel O’Connell was promoted to the rank of colonel, and to that of major-general on the 22nd of July, 1830. In 1838, Major-General O’Connell was appointed to the command of the troops in New South Wales, which appointment he held from December of that year until December, 1847.