Hackett, Sir Thomas (d. 1706), merchant, banker, and Jacobite mayor of Dublin, was the second of two sons of James Hackett, of a family long established in Co. Tipperary. His mother was Alson Hackett, daughter of Jasper White, and he had three sisters. He was engaged in foreign trade out of Dublin by 1666, and his business associates included Sir Daniel Arthur (qv) and George Macartney (qv). He acquired much property including houses on Merchants Quay in Dublin, where he lived, and land in counties Dublin, Westmeath, and Limerick. He was a banker, and members of the Old English nobility and gentry were prominent among his debtors. He was connected by marriage and other ties to important catholic merchant families such as the Arthurs and the Creaghs (one of whom – Sir Michael (qv) – succeeded Hackett in the mayoralty of Dublin in 1688), and was a friend and protégé of Richard Talbot (qv), earl of Tyrconnell, lord deputy to King James II (qv).
He enjoyed brief political prominence during the reign of James, who in October 1687 made him lord mayor of Dublin (1687–8) and knighted him. William King (qv), the protestant chronicler of Jacobite Ireland, gives a very hostile account of Hackett's mayoralty, claiming he did ‘many brutish and barbarous things’ to protestants. These included arresting a nephew and son-in-law of Archbishop Michael Boyle (qv) for ‘refusing to contribute to the maintenance of two begging friars’, and committing ‘the officers of Christ-Church, Dublin, to the stocks, because he fancied they did not make the bells ring merrily enough for the birth of the prince of Wales’. Sir Thomas sat for the borough of Portarlington in King James's Irish parliament in 1689 and, probably in the same year, was deputy lieutenant and a justice of the peace in Dublin.
The war and defeat of James's cause were disastrous for him; he was on the first list of Jacobites outlawed and most of his properties were seized. In addition he became deeply indebted, probably because of the disruption to his trading interests, and he claimed in 1696 that he had lost about £13,000. He had a pass in 1694 to go to Holland, where his activities were monitored by English government officials who believed him to be an agent of the Jacobite court at Saint Germain. He and another merchant called Hackett, in Rotterdam during 1694–6, were suspected of transmitting correspondence between England and Saint Germain, and providing passage to England for Jacobite spies and deserters from the English armies in Flanders. In 1698 there was a warrant for Sir Thomas's arrest, for returning to England without licence.
He attended hearings of the trustees for the forfeited lands in Dublin in 1700 as a witness, and as such was granted legal protection by the trustees. When one of Hackett's creditors tried to have him arrested by the under-sheriff of Dublin, the trustees had the official arrested resulting in a jurisdictional dispute with the Irish courts. In 1705 his creditors petitioned the Irish house of commons for relief and, with Hackett's consent, an act appointing trustees to sell his estates was passed. The act's preamble noted that Hackett, then imprisoned in Dublin for debt, was also a substantial creditor but because of his outlawry was unable to sue for money owed to him. He died in 1706 (will proved 20 July). His wife was Mary, daughter of John Arthur, perhaps the husband and cousin of Dymphna, daughter of Dr Thomas Arthur (qv). Dame Mary was alive in 1705, and Sir Thomas was survived by a son and five daughters.
The disposal of Sir Thomas's assets was a complex affair, and the occasion of repeated complaints to the Irish and British parliaments. The Irish parliament passed a second, explanatory, act in 1707, and the Hackett children petitioned for the preservation of their entitlements. The guardian of Sir Thomas's son John was very hostile to the trustees, who included Robert Rochfort (qv) and William Conolly (qv). He sought a private act of the British house of commons in 1708 to curb the trustees, but proceedings were abandoned after a committee hearing ‘was the occasion of drawing several papists together about this house, contrary to her majesty's proclamation’ (Commons jn. [Eng.]., vol. xv, 600). He returned to the British commons in 1709 but the trustees petitioned against his bill, which was not reported. His petitions to the Irish house of commons for a private act, in 1710 and 1711, were also fruitless. The trustees meanwhile had sold the Hackett estates in 1708–9, raising over £15,000. John Hackett (d. 1760), was an ‘infant’ in 1708 and a minor in 1709 and 1710. He probably was not the John Hackett returned for the borough of Mullingar to the Irish house of commons in 1727, and declared not duly elected in 1728.
Sir Thomas's elder brother, George, married a Dutch woman, Katherine Drull, and had a son and a grandson who were in the service of the house of Orange. The Hackett of Rotterdam associated with Sir Thomas in the 1690s may have been a relation; and may have been the John Hackett, merchant, of Rotterdam who in 1712 was an important link between the exiled Jacobite court and its followers in England (including the Irishman John Plunket (qv)).Two of Sir Thomas's daughters were in France in the reign of Anne. One of them, Catherine, returned in 1711, and outraged her Irish kinsman and host, John Arthur of Cabraugh, Co. Dublin. He alleged that she, nearly forty years of age, seduced and contracted a clandestine marriage with his sixteen-year-old son Benedict – but not before divesting herself of her own property in favour of her siblings.
Glenstal Abbey, Murroe, Co. Limerick, has manuscripts relating to Sir Thomas's landholdings in that district, and a unique printed copy of the act of 1705. The Gilbert Library in Pearse Street, Dublin, has records of the trustees appointed to sell Hackett's lands, and Hampshire Record Office has a copy of John Hackett's British bill.