On 22 February 1781 he was appointed midshipman on board the East India Company's sloop of war Swallow, and in July that year obtained an infantry cadetship; was made ensign in the Bengal army on 9 May 1782 and lieutenant on 6 January 1785.
His gallantry at the sieges of Bursaar and Jeswarree in January 1804 led to his appointment to supervise the operation irregular force of two thousand men, under Shaik Kurub Ali, in the interior of Bundelkund.
On 1 July he commanded two brigades of irregulars in another attack on Raja Rām and a force of sixteen thousand Bondeelas and Naghas on the fortified hills of Thanah and Purswarree.
On 1 November 1805 he was appointed commissary of supplies by Lord Lake, and, on the breaking up of the army on 1 June 1806, rejoined his regiment, and on 25 April 1808 attained the rank of major.
[1] Colonel Gabriel Martindell, who commanded in Bundelkund, made O'Halloran his military secretary; and his conduct at the head of the first battalion 18th native infantry at the siege of the fortress of Adjeghur was specially noticed.
He died at his residence in Connaught Terrace, Hyde Park, London, on 3 November 1843, from the effects of a street accident, causing fracture of the neck of the thigh-bone.
[1] O'Halloran married, in 1790, Frances, daughter of Colonel Nicholas Bayly, M.P., of Redhill, Surrey, late of the 1st foot-guards, and brother of the first Earl of Uxbridge, by whom he had a large family.