Cairnes, William Plunket (1857–1925), businessman and banker, was born 1 April 1857, eldest among four sons and three daughters of Thomas Plunket Cairnes (1830–94), DL, of Stameen, Co. Louth, and Monkstown Park, Co. Dublin, brewer and godson of Thomas Span Plunket (qv) (Lord Plunket and bishop of Tuam) and Sophia, daughter of Charles Gaussen. Thomas Plunket Cairnes was a younger brother of John Elliott Cairnes (qv) (1823–75), economist and professor of political economy at TCD and later at London. William Plunket Cairnes's grandfather, William Cairnes, served as an apprentice brewer at Castlebellingham brewery, before founding his own brewery at Drogheda in 1825. The Cairnes family was originally from Scotland, as were the distillery owning Jamesons with whom they intermarried.
Educated at Cheltenham College and Trinity Hall, Cambridge (1876–80) (where he was a senior wrangler), William Plunket Cairnes graduated BA (1880) and entered the family brewing business, of which his father was then chairman and official head. However, William was responsible for much of the running of the firm, and in 1890 he helped to oversee the amalgamation of the Drogheda business with the brewery at Castlebellingham and the flotation of the new enterprise, with a share capital of £265,000. On the death of his father (1894) he became chairman and principal shareholder of the Drogheda & Castlebellingham Brewery Co. Ltd. Appointed a director of the Great Northern Railway (1896), deputy chairman (1909), and chairman (February 1923), he was also a director of the Newry & Greenore Railway, the City of Dublin Junction Railway (‘Loop line’), and the Strabane & Letterkenny Railway, as well as the Liverpool and London and Globe Insurance Co. He served as a director of the Bank of Ireland (1908–25) and was governor 1920–22. Against the appointment of the Bank of Ireland as the financial agents of the provisional government in January 1922, Cairnes argued that the same organisation could not combine the duties of a state bank with acting as an ordinary, competitive, commercial and industrial bank.
Closely identified with the working of several charitable institutions including the sanatorium at Newcastle, Co. Wicklow, Cairnes took an active part in church and social work throughout Co. Louth and Co. Meath, and was particularly interested in education and housing. Under the government's Housing (Building Facilities) Act, 1924, he built a terrace of twelve houses on Cromwell's Mount, Drogheda, Co. Louth. His home farm at Stameen was a model of its kind. After a brief illness he died 18 December 1925 at his Dublin residence, 6 Raglan Road, leaving an estate valued at £102,828. For his funeral a special train left Amiens Street station for Drogheda.
He married (1 December 1886) Alice Jane (d. 27 December 1952), daughter of Maj.-gen. James Sturgeon Hamilton Algar. They had two sons, one of whom was Lt-col. Cairnes, a director of the Great Southern Railway Co. The family lived at Stameen, Drogheda, Co. Louth.