John O'Brien (6 December 1794 - 6 February 1855) was an Irish MP who represented Limerick City in the UK Parliament 1841–1852.
James died on 21 February 1806, and Margaret subsequently married Cornelius O'Brien (1782-1857), MP for Clare 1832-47 and 1852–57.
He was elected MP for Limerick City in 1841 (Whig Party), and was re-elected in 1847 (Repeal Association); on both occasions as first member in that two-member constituency.
Their fifth son, Peter, became a lawyer and judge, and served as Lord Chief Justice of Ireland.
John's youngest brother, James, their parents' fourth son, was a lawyer, judge and politician who represented Limerick City as MP from 1854–1858.