Murrough O'Brien was born in 1726 to the
Hon. James O' Brien and Mary Jephson in
Drogheda. James' brother (and Murrough's uncle) was
Henry O'Brien, 8th Earl of Thomond, whose heir was
Percy Wyndham-O'Brien, 1st Earl of Thomond (c. 1713 – 1774), brother of
Charles Wyndham, 2nd Earl of Egremont (1710–1763) of
Petworth House. He joined the
Grenadier Guards and was an officer in Germany, where he carried colours at the
Battle of Lauffeld in 1747. He retired in 1756 and entered the
Irish House of Commons for
County Clare in the following year. He represented the constituency until 1761 and sat then as
Member of Parliament (MP) for
Harristown until 1768. Because of his support for the
Act of Union of Great Britain and Ireland, on 29 December 1800 he was created
Marquess of Thomond in the
Peerage of Ireland, with a special remainder to his younger brother, and
Baron Thomond, of
Taplow Court in the
County of Buckingham in the
Peerage of the United Kingdom on 2 October 1801 (which title allowed him to sit in the United Kingdom
House of Lords), but this time with no special remainder. He had a close relationship with King
George III. In 1783 he was one of the Founding Knights of the
Order of St Patrick. His Irish seat was at
Rostellan, near
Cork city. He was a drinker, called a "'six bottle man", and a gambler. He was a keen farmer and oversaw the enclosure of lands around Taplow and mechanisation. ==Marriages and children==