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O'Brien was the younger son of an Irish baronet. He inherited his mother's family property at Cahirmoyle, co. Limerick, and consequently took the additional surname Smith. After his education at Trinity College, Cambridge, O'Brien was nominated by his father for the pocket borough seat of Ennis. He entered parliament in 1828. Although himself a protestant, he joined the Catholic Association. He brought in an unsuccessful Irish Poor Law bill in 1831. In the same year, he left his Ennis seat and moved to co. Limerick. He was elected as an Independent Liberal M.P. for the county in 1835, a seat he retained until 1849.
In 1843 O'Brien joined Daniel O'Connell's Repeal Association, which campaigned for repeal of the Act of Union of Great Britain and Ireland. He was briefly imprisoned in 1846 for refusing to serve on a Scottish railway committee. In the same year, O'Brien split the Repeal Association and became leader of the Irish Confederation.
In 1848, in response to the revolution in France, he demanded the formation of a national guard in Ireland and advised the Irish to arm themselves. He was tried for sedition, but the jury could not reach a verdict and he was not convicted. In the summer of 1848 O'Brien half-heartedly led an insurrection in the southern counties of Ireland. Convicted of high treason at Clonmel courthouse, his sentence of death was commuted to transportation to Van Diemen's Land [Tasmania].
O'Brien received a conditional pardon in 1854 and moved to Brussels, where he published Principles of Government, or, Meditations in Exile. In 1856 he received a full pardon and travelled back to Ireland. In his later years he wrote articles, managed his estates, and travelled extensively in Europe and America.