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. 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. 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

The North Irish Horse was a yeomanry unit of the British Territorial Army raised in the northern counties of Ireland in the aftermath of the Second Boer War. Raised and patronised by the nobility from its inception to the present day, it was one of the first non-regular units to be deployed to France and the Low Countries with the British Expeditionary Force in 1914 during World War I and fought with distinction both as mounted troops and later as a cyclist regiment, achieving eighteen battle honours. The raising of Militia units in Ireland commenced with the "Militia Act 1793", which in Ireland was used in conjunction with the compulsory disbandment of Lord Charlemont's Irish Volunteers, who had become a political entity and "out of the scope of official influence". The scope of the Militia was broadened by an act of the Dublin Parliament in 1796, which led to the raising of forty-nine troops of cavalry, later renamed yeomanry. A troop normally consisted of a captain, two lieutenants (commissioned by the Lord Lieutenant of Ireland) and forty men, along with a permanent sergeant and trumpeter. Troops were grouped together under the command of a regular army brigade major. The force was known collectively as the "Irish Yeomanry". Each man provided his own horse. The falling need for this force eventually led to its disbandment in 1834. With the advent of the Boer War, a parliamentary decision was taken to raise squadrons of Yeomanry