Charles O'Brien, (17 March 1699 – 9 September 1761), titular 6th Viscount Clare and later titular 8th Earl of Thomond, was an Irish military officer in French service (he was made a Marshal of France), known to posterity as the Maréchal de Thomond.

Charles O'Brien was the son of Charles O'Brien, 5th Viscount Clare and Charlotte Bulkeley, the sister of Anne Bulkeley, second wife of James FitzJames, 1st Duke of Berwick and Marshal of France.

[1] When his cousin, Henry O'Brien, 8th Earl of Thomond, offered the Thomond estates to Charles on the condition of his conversion to Protestantism, he refused, and so he willed them to the young son of William O'Brien, 4th Earl of Inchiquin, Murrough, with remainder to Percy Wyndham.

[2] Murrough's death in 1741 caused the reversion to become effective, with the estates leaving O'Brien hands.

They had three children:[1] Lord Thomond died on 9 September 1761, aged 62, at Montpellier, France.