Molyneux, Samuel (1689–1728), scientist and politician, was born 18 July 1689, son of William Molyneux (qv) and his wife, Lucy Molyneux (née Domvile), at Chester. His mother died in 1691 and his father, who followed the educational precepts of John Locke in rearing his only surviving child, in 1698. The boy, by his father's account precocious, was then taken care of by his uncle Thomas Molyneux (qv), and entered TCD in 1705, graduating BA in 1708 and MA in 1710. At the college he received private tuition from George Berkeley (qv), who was deeply impressed by the youth and who became his close friend. Molyneux developed an early interest in natural philosophy and while still an undergraduate was prominent in the revival in 1707 of the Dublin Philosophical Society, which elected him secretary. Among his correspondents was the Gaelic scholar Roderick O'Flaherty (qv), whom he travelled to see in Galway in 1709; his journal of this trip survives. In the same year he left Dublin to spend two years carrying out improvements on the estate he inherited at Castle Dillon in Co. Armagh.
In 1712 he left for England, where he visited scientific instrument makers in London, the observatory at Greenwich, and the universities, and was elected a fellow of the Royal Society. Soon afterwards he met John Churchill (qv), duke of Marlborough, and his wife, at Antwerp and in May 1714 became their agent at the court of Hanover, at a time when it was a centre of intrigue by English political factions. On the death of Queen Anne he accompanied the new king, George I, to England. He was appointed secretary to the prince of Wales and became a member of the English house of commons (he served from 1715 to 1722 and 1726 to his death) and of the Irish privy council; he was later a member of the Irish parliament (for TCD) from 1727 to his death. Molyneux's duties to the prince of Wales did not involve much Irish business, but when the post of chancellor of TCD became vacant by the attainder of the duke of Ormond (qv), he was able to assist his friend the archbishop of Dublin, William King (qv), in his project to have the prince assume the office. When the prince acceded to the throne as George II in June 1727 Molyneux's post as his secretary was superseded, and the following month he was appointed one of the lords of the admiralty.
Between 1723 and 1727 he undertook his most notable scientific work, much of it in collaboration with his friend James Bradley, principally in the fields of astronomical observation and the design and manufacture of optical instruments. In 1726 he was involved in an uncharacteristic episode, the celebrated case of the Surrey woman Mary Tofts who was supposed to have given birth to rabbits. An account was published by Nathanael St André, a Swiss-born anatomist at the court of George I, who went so far as to assert that he had personally delivered some of the rabbits. St André lost his entrée to the court when the affair was exposed as a hoax, and Molyneux, who had been unwise enough to endorse the account, was exposed to some ridicule.
He married in 1717 Lady Elizabeth Capel, the eldest daughter of Algernon Capel, the second earl of Essex, and his wife, Mary Capel (née Bentinck). Her father was a general and a member of the household of King William III (qv), while her mother was a daughter of the earl of Portland; her grandfather, the first earl of Essex (qv), had been a lord lieutenant of Ireland and her great-uncle Henry Capel (qv) a lord deputy. The latter's widow, Dorothy Capel, on her death in 1721 left Elizabeth a considerable fortune as well as Kew House, which was the scene of her husband's astronomical work. Their marriage was without issue.
In 1728 Molyneux collapsed in the English house of commons; St André attended him in the capacity of physician but he survived only a few days, dying at his home on 13 April; his widow went off the same night with St André. The circumstances of his death gave rise to rumours of the most sinister import; the charge, which appeared in print, was that St André had, with the connivance of Molyneux's wife, deliberately poisoned his patient with opium. In 1730 St André and Lady Elizabeth married.
Some of Molyneux's correspondence and other papers are preserved at the Civic Centre Archives in Southampton and at TCD; Gillespie lists his published works.