O'Brien was born in England the second of four children, and oldest son, to Edward O'Brien, 14th Baron Inchiquin and first wife Emily A'Court, at Belmore near Bishop's Waltham, Hampshire [REF].
O'Brien was commissioned into the Royal Irish Rifles in 1885, transferring in 1886 to the English Rifle Brigade regiment in which he served until 1893 [REF].