Stewart, William (1650–1692), 1st Viscount Mountjoy , army officer, was born six weeks after the death of his father, Sir Alexander Stewart, at the battle of Dunbar in September 1650. His grandfather Sir William Stewart (d. 1646/7) went to Ireland in 1608 as captain of 100 foot soldiers, acquired land in Co. Wexford in 1612 and was granted large tracts of land in counties Donegal and Tyrone under the Ulster plantation scheme. He was created a baronet in 1623 and was member of parliament for Donegal county in 1613 and 1634. His mother, Catherine (d. 1714), a daughter of Sir Robert Newcomen, later married Sir Arthur Forbes (qv), future 1st earl of Granard, who was granted the wardship of the young boy on 16 February 1660.
William Stewart was one of the commissioners appointed to settle the claims of the 1649 officers in October 1675. From 1677 to 1684 he captained the king's company regiment of guards in Ireland and he was appointed custos rotulorum for Co. Donegal in 1679. He was raised to the peerage in May 1683 as Baron Ramelton and 1st Viscount Mountjoy. In April 1684 he became master general of the ordnance in Ireland for life, selling his commission as captain of the guards in order to finance the purchase of this office. He raised and commanded a regiment of infantry in March 1685, which was primarily based in the Donegal–Tyrone region. Mountjoy went to England in 1686 to inform James II (qv) about the poor condition of the ordnance in Ireland and, from there, he travelled to Hungary, where he was seriously wounded taking part in the siege of Buda. On his return to Ireland in 1687, he was promoted to the position of brigadier general with an income of £497 10s. per year.
In 1688 Mountjoy's predominantly protestant regiment was based near Derry. In November 1688 Lord Deputy Tyrconnell (qv) ordered the regiment to Dublin, intending to ship them to England. He instructed the earl of Antrim (qv) to raise a catholic force, to replace Mountjoy's regiment, and garrison the city. When the citizens refused to admit Antrim's catholic troops, however, Tyrconnell was forced to send Mountjoy back to Derry with six companies of his regiment in order to secure the city. On his arrival in December 1688, he pacified the area and negotiated terms with the citizens, whereby they agreed to admit two protestant companies into the city with Mountjoy as governor. Tyrconnell, distrustful of Mountjoy, whose local influence marked him out as a possible leader of the Ulster protestants, recalled him to Dublin. Despite the advice of friends, he returned to Dublin where he was persuaded by Tyrconnell to accompany Sir Stephen Rice (qv) to St Germain in France, ostensibly to urge James II to make terms with William of Orange (qv), as Ireland could not hold out against an Anglo–Dutch invasion. When the two men left Ireland on 10 January 1689, however, Rice brought with him a secret letter to James II, advising him that Mountjoy was not to be trusted and should be arrested. James II was willing to let him go but the king of France, deciding that he was too important as a potential leader of the protestants, had him detained in the Bastille. Although imprisoned in France, Mountjoy was attainted by the Irish parliament, on 7 May 1689, for non-attendance and deprived of his lands. Various attempts to have him released proved unsuccessful until April 1692, when he was exchanged for Richard Hamilton, who had been captured at the battle of the Boyne. On his release, Mountjoy went to Flanders to thank William III for procuring his freedom but on 3 August 1692 he was killed by a cannonball at the battle of Steinkirk.
Mountjoy married Mary Coote, daughter of Richard, 1st Baron Coote of Coloony, and sister of Thomas Coote, appointed a judge of the king's bench in 1693. They had six sons and two daughters: William, Alexander, Richard, Arthur, Charles, James, Mary and Catherine. William (d. 1728), who succeeded his father as 2nd Viscount Mountjoy, married, in 1696, Anne (d. 1741), daughter and heir of Murrough Boyle (qv), 1st Viscount Blessington, thus acquiring the Boyle estates in counties Kildare and Wicklow. Mountjoy's third son, Alexander, who was admitted to the Middle Temple in London on 29 April 1696, married Mary, daughter of William Tighe, member of an important civic family in the city of Dublin.