Dillon, Sir James (c.1600–c.1667), army officer, was the eighth son of Theobald, 1st Viscount Dillon (qv) of Costello-Gallen (d. 1624/5) and Eleanor (d. 1638), the daughter of Sir Edward Tuite of Tuitestown, Co. Westmeath, and widow of William Tuite of Monilea. His father, Theobald, was appointed chief serjeant and general collector and receiver for Connacht and Thomond in May 1582. In 1585 he acquired ownership of the entire barony of Costello in Co. Mayo, and also procured additional lands in Co. Roscommon. Theobald Dillon was knighted by the earl of Essex in 1599 and created Viscount Dillon of Costello-Gallen in March 1622. As he was predeceased by his eldest son, Sir Christopher Dillon, the title passed to his fourteen-year-old grandson Sir Lucas Dillon in 1624.
In 1627 James Dillon was selected as one of the commissioners delegated to raise money for the army in Ireland. He became a freeman of Dublin in 1632 and was returned as member for Co. Westmeath to the 1634 parliament. He was listed as a captain in the regiment of Owen Roe O'Neill (qv) at Cambrai in 1636, being the first Dillon to serve in a foreign army. He returned to Ireland later that year and, in 1640, was again returned to parliament as member for Co. Westmeath. With the creation of the ‘new’ Irish army of Thomas Wentworth (qv) in 1640, Dillon served as a captain in the regiment of Charles Coote (qv). When the army disbanded, he was one of nine officers authorised by Charles I to recruit regiments from among the disbanded soldiers for military service in Spain. Dillon lost the £1,000 he invested in the project, when it was halted because of growing political tensions between king and parliament. In the early autumn of 1641, before the outbreak of rebellion, he was involved in discussions with Rory Maguire (qv) regarding the proposed ‘colonels’ plot’ to seize Dublin castle, though he later appears to have withdrawn his support for the conspiracy.
He was not in Dublin when rebellion broke out in October 1641 and the lords justices seem to have been unaware of his involvement in the plot, as they sent him arms and appointed him joint governor of Longford, together with his namesake Sir James Dillon, son of the earl of Roscommon (generally referred to as the younger, but it can be difficult to differentiate between the two).
Dillon subsequently became active in the rebellion in counties Longford and Westmeath. In January 1642 he escorted some protestant residents of Athlone to safety though he kept the lord president of Connacht, Lord Ranelagh, confined in Athlone until July 1642, when he was forced to lift the siege. On 22 June 1642 he was indicted for rebellion and expelled from the Irish house of commons. Dillon was with Preston (qv) at the siege of Birr in January 1643 and the following March his regiment was in the area of the battle of Ross, though it was not engaged there. Throughout the 1640s he was a member of the confederate council of war for Co. Westmeath. Despite the cessation of hostilities, agreed in September 1643, Dillon wrangled with Ormond (qv) for control of Athlone. In 1644 his regiment of 1,000 men was chosen to form part of the futile expedition led by Castlehaven (qv) to Ulster, and he was also involved in reducing Co. Roscommon. In 1645 Dillon was listed as a commander for Connacht and his regiment formed part of Preston's army that campaigned in the region in 1646. Part of his regiment also served under Oliver Fitzwilliam (qv) in Co. Roscommon. Dillon was personally involved in the siege of Roscommon town, following which he was appointed governor of Jamestown in the same county.
In 1647 Dillon was one of the confederate officers who kept his army together in the hope of taking it into French service. Although Ormond recommended him to Dumolin, the French agent, Dillon was prevented from taking his men into the French service by a lack of funds. His cavalry troop was at the battle of Dungan's Hill in August 1647 and, in the summer of 1649, his regiment formed part of the force led by Inchiquin (qv) that captured Drogheda and Dundalk. Two hundred members of his infantry regiment were present at Drogheda when it was stormed by Oliver Cromwell (qv), though it is unlikely that Dillon was personally in the town. Ormond appointed him joint governor of Athlone, where he was active in organising the region's defences. On 18 June 1651, with two parliamentary armies advancing against him, Dillon surrendered the town. The surrender greatly weakened the royalist line of defence behind the Shannon and there were rumours of possible treachery. In 1652 he agreed to lay down his arms and remain in Mullingar.
Dillon was exempted from pardon by the act of settlement and went into Spanish service, bringing 575 Irish soldiers to Bordeaux. On 26 May 1653 he surrendered the fort of Ormont to the French commander, the duke of Vendôme. For this service, Dillon was well rewarded by the French, being promoted to brigadier general and given the command of a regiment in their army. On 16 June 1653 the Dillon regiment was founded and it served in Flanders until 1663. It was disbanded on 29 February 1664 upon inaccurate reports of the colonel's death. The exact date of Dillon's death remains uncertain, as he was in receipt of a pension of £500 from Charles II which was still being paid in 1669.
He seems to have returned to Ireland, however, as in 1666 he carried draft bills to London for the English secretary of state, Lord Arlington, and he was also a signatory of the loyal catholic remonstrance to the king. In 1667 he became involved in an innovative business venture, when he sought a licence to manufacture a new type of fuel. Although he married twice, Dillon left no surviving children. Some time before 1638 he married Elizabeth (d. before 1653), daughter of Thomas Plunkett of Rathmore, Co. Meath, with whom he had two sons, Ulick and James, who both predeceased their father. His second marriage, to Mary (d. before Jan. 1665), a daughter of Sir Roger Jones of Sligo and widow of Major John Ridge of Co. Roscommon, was childless.