O'Brien, Cornelius (c.1782–1857), landlord, lawyer, and MP, was born at Birchfield, Liscannor, Co. Clare, the third son of Henry O'Brien of Birchfield and Ennis and his wife Hellen O'Callaghan, of Kilgorey, Co. Clare. Educated in Ennis, he entered the King's Inns in 1803, qualified as an attorney in 1808, and was apprenticed to Cornelius O'Callaghan at the courts of chancery and common pleas and exchequer. He was also a proctor, solicitor, and magistrate for Co. Clare. Not one to take insult lightly, he fought a duel when aged nineteen and in 1810 was convicted of the manslaughter of Francis Drew in a duel. He was sentenced to six months' imprisonment, but such was the force of his plea that the sentence was abridged to a week. At the age of 65 he was called to a third duel but this never took place as both parties were bound to the peace on assurances of £500 each. Involved in local politics, O'Brien supported catholic emancipation, repeal of the union, the secret ballot, and tenant right. A member of the committee which selected Daniel O'Connell (qv) to run in the 1828 by-election in Clare, he was himself elected repeal MP for Co. Clare (1832–47, 1852–7). However, he made little impression at Westminster: Lord Palmerston commented that he was the best Irish MP who ever sat in parliament because in twenty years he had never opened his mouth.
O'Brien's holdings in Clare were extensive and included his estate at Birchfield and the Toonagh Estate. He owned freeholds valued at £50 in 1829 and was admired as an improving resident landlord. In 1855 he held 9,679 acres spread across seventeen parishes and thirty-six townlands. Occupied by 275 tenants, his property encompassed 2,960 acres in East Clare and 6,719 acres in West Clare (Griffith Valuation). According to his testimony to the Devon Commission (1844) he supplied his tenants with employment, clothing, and education as a reward for their improvements to the land. Official visitors and tourists noted the number of white-washed and comfortable cottages scattered over the Birchfield estate even during the Famine, although he angrily denied accusations of favouring his tenants in allocating relief work. His numerous building projects included the building of Birchfield House before 1816 and the re-location and upgrade of St Bridget's Well at Kilmacrehy from higher up the hillside to its present location beside the road where a well-house and rustic seating were also erected. He also facilitated the building across the dangerous ford of the river between Lahinch and Liscannor of a fine stone bridge of three arches which still stands and is known as O'Brien's Bridge. An early promoter of tourism in the area, he erected a viewing tower on the Cliffs of Moher together with stables and an iron picnic table and helped make the cliffs accessible, safe and attractive to strangers who were frequently entertained at Birchfield House. He also erected a little Gothic well-house at the Reliever's Well. A carving of the O'Brien crest appears both on this building and the tower on the cliffs. In 1845 he built a national school beside the main road to St Bridget's well.
O'Brien also ran a legal business in Dublin, initially at 32 Great Georges Street North but by 1842 had moved to 20 Summerhill and formed a partnership with William Henry McGrath, who became his son-in-law in October 1852. He was chairman of the Ennistymon Poor Law Union from its formation on 25 August 1839 and chairman of the Westminster Loan and Investment Stock Company in the 1840s. Advancing age and ill health forced his retirement from parliament in March 1857, two months before his death on 30 May 1857.
In 1816 Cornelius O'Brien married Margaret (née Long) (d. 1839), widow of James O'Brien (d. 1806) of Ballynalackan, with whom she already had four sons, including John O'Brien (1794–1855), MP for Limerick city (1841–52) and James O'Brien (1806–81), MP for Limerick city (1854–8) and a judge of the court of queen's bench. Proof of a second marriage is elusive but Cornelius O'Brien fathered at least two children (George and Mary Ann) with Mary O'Brien. George (d. 1867), his only surviving son, inherited Birchfield and was a captain in the Clare militia and a magistrate. Before the beginning of December 1867 Birchfield was sold to Cornelius O'Brien's son-in-law Cornelius Keogh, high sheriff of Co. Clare. There are no known portraits of Cornelius O'Brien but he is honoured by two memorials: O'Brien's Tower on the Cliffs of Moher and the testimonial monument erected near St Bridget's Well.