| Office: | Baron Inchiquin |
| Term Start: | 9 April 1900 |
| Term End: | 9 December 1929 |
| Predecessor: | Edward O'Brien |
| Successor: | Donough Edward Foster O'Brien |
| Honorific-Prefix: | The Right Honourable |
| Nationality: | British |
| Allegiance: |  United Kingdom |
| Father: | Edward O'Brien, 14th Baron Inchiquin |
| Children: | 6, including Donough Edward Foster O'Brien, 16th Baron Inchiquin and Phaedrig Lucius Ambrose O'Brien, 17th Baron Inchiquin |
| Relatives: | Conor Myles John O'Brien, 18th Baron Inchiquin (grandson) |
| Education: | Eton College |
| The Lord Inchiquin | |
| Death Place: | London, United Kingdom |
| Birth Place: | Bishop's Waltham, United Kingdom |
| Birth Date: | 21 June 1864 |
| Birth Name: | Lucius William O'Brien |
| Serviceyears: | 1885–1893 |
| Rank: | Lieutenant |
Lucius William O'Brien, 15th Baron Inchiquin (21 June 1864 – 9 December 1929) was the England-born holder of a hereditary peerage in the Peerage of Ireland, as well as Chief of the Name of O'Brien and Prince of Thomond in the Gaelic Irish nobility.
O'Brien was born in England the second of four children, and oldest son, to Edward O'Brien, 14th Baron Inchiquin and first wife Emily A'Court, at Belmore near Bishop's Waltham, Hampshire. He was educated at Eton College.
O'Brien was commissioned into the Royal Irish Rifles in 1885, transferring in 1886 to the English Rifle Brigade regiment in which he served until 1893. He was later Honorary Colonel of the Clare Royal Field Artillery Reserve Volunteers.
Politically a Conservative, O'Brien unsuccessfully stood for the British House of Commons by contesting the Eastern Division of County Clare in 1885.[1] He was State Steward to the Lord Lieutenant of Ireland in 1895. He succeeded his father's peerage in 1900, serving hence as an Irish Representative Peer in the House of Lords. In 1921 he was appointed to the Senate of Southern Ireland which was abolished the next year by the formation of the Irish Free State.
He was Deputy Lieutenant and Justice of the Peace for County Clare, of which county he was High Sheriff for 1898. He was also a Justice of the Peace for the county of Salop, Shropshire.
On 14 January 1896, Inchiquin married, at Richard's Castle, Ethel Jane Foster, daughter of Johnston Jonas Foster, of Moor Park near Ludlow, Shropshire, which became his English residence. Together they had six children:[2]
Lord Inchiquin died on 9 December 1929, aged sixty-five, and was succeeded by two of his sons as barons.[2]