麥當奴
O'Halloran was born at Adelaide, South Australia, was the son of Captain William Littlejohn O'Halloran and his wife Eliza Minton O'Halloran. He was educated at private schools, and entered the South Australian Civil Service, receiving his first appointment from Sir Richard Graves MacDonnell, the then Governor. After passing ten years in the Audit Office, and rising to the third position in that department, he was promoted to the clerkship of the Executive Council, being at the same time gazetted Clerk to the Court of Appeals. These offices he retained until 1871, and, in addition, acted for a while as private secretary to the Right Hon. Sir James Fergusson, 6th Baronet, Governor of the colony. In consequence of certain reductions in the estimates, which involved several departmental changes, O'Halloran retired from the Government service in 1871, with compensation for loss of office, the Governor in Council recording his testimony to the honourable manner in which he had discharged his duties. Soon after his retirement O'Halloran came to England, was elected a Fellow of the Royal Geographical Society and of the Royal Colonial Institute, and found occupation in the city of London, where he filled secretarial appointments for a period extending over seven years. In 1872 he acted—in conjunction with Dr. C. W. Eddy—as honorary secretary and a member of the committee for celebrating the opening of telegraphic communication with Australia.