O'Brien was commissioned into the 14th/20th Kings Hussars of the British Army in 1963. He served as a troop commander in Benghazi, Tripoli, Cyprus on Operation Tosca, Tidworth, Paderborn (as assistant adjutant), Singapore, and Tidworth as adjutant of the regiment. He became Aide-de-Camp to Commander British Forces Gulf in Bahrain and left the army in 1975, retiring with the rank of Captain.
Wikipedia: Baron Inchiquin (Irish: Barún Inse Uí Chuinn) is one of the older titles in the Peerage of Ireland. It was one of two titles created on 1 July 1543 for Murrough O'Brien, Prince of Thomond, who was descended from the great high king Brian Boru. The grant of the English titles was conditional upon the abandonment of any Irish titles, the adoption of English customs and laws, pledging of allegiance to the Crown, apostasy from the Catholic Church, and conversion to the Church of England. Murrough was made both Earl of Thomond in the Peerage of Ireland, with remainder to his nephew Donough O'Brien and Baron Inchiquin, with remainder to his male heirs.
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Dromoland Castle, The ancestral seat
Barony of Inchquin, created by Henry VIII