 O'Brien was invested as a knight on 1 July 1604 in Leixlip, Co. Kildare, Ireland and held the office of member of the Irish House of Commons for Clare from 1613 to 1614.O'Brien was also a member of the Supreme Council of the Catholic Confederates and was created 1st Viscount Clare and 1st Baron Moyarta in 1662, as a reward for loyalty to the Crown.He had been a strong supporter of King Charles I of England during the English Civil War, and as a result much of his land was confiscated; but he recovered a good deal of it at the Restoration of Charles II together with the peerage.