John O'Brien (6 December 1794 - 6 February 1855) was an Irish MP who represented Limerick City in the UK Parliament 1841–1852. O'Brien was the eldest son of James O'Brien of Limerick and Margaret Long, daughter of Peter Long of Waterford. James died on 21 February 1806, and Margaret subsequently married Cornelius O'Brien (1782-1857), MP for Clare 1832-47 and 1852–57. John's youngest brother, James, their parents' fourth son, was a lawyer, judge and politician who represented Limerick City as MP 1854–1858.