Dillon, Thomas (1615–1673/4), 4th Viscount Dillon of Costello-Gallen, army officer, was born in March 1615, second son of Sir Christopher Dillon of Ballylagham, Co. Mayo, lord president of Connacht, and Lady Jane, eldest daughter of James Dillon, 1st earl of Roscommon. He was a grandson of Sir Theobald Dillon (qv), 1st Viscount Costello-Gallen, and a nephew of Sir James Dillon (qv), army officer and confederate. Raised a catholic, he converted to protestantism in 1630 when, aged fifteen, he succeeded to the title on the death of his infant nephew Theobald, the 3rd viscount. The ward of his estate was granted to his uncle Sir Lucas Dillon (qv) and Lord Wilmot (qv), until he came of age and received livery of his lands on 15 March 1636.
In November 1641 he was appointed joint governor of Mayo with Miles Bourke (qv), Lord Mayo and, for a time, he succeeded in keeping the county free of rebels. In the same month he was chosen, together with his brother-in-law Lord Taaffe (qv), to go on a secret mission to the king in England on behalf of members of the Irish house of lords and present him with a remonstrance. A storm forced their ship to land in Scotland, from where they set out for England. By order of the house of commons, they were arrested at Ware as rebel agents and taken to London. They were held by parliament for several months, before they succeeded in escaping to join the king at York. Charles I refused to return them to parliamentary custody and granted Dillon a command of horse in the royalist army.
Dillon returned to Ireland in July 1644, when he was appointed lieutenant general of the royalist army and joint president of Connacht with Lord Wilmot. Based at Athlone in 1645, he sought to prevent the Scots army from wasting the province. In 1646 he went to Dublin and, as a member of the council led by Ormond (qv), he was a signatory of the peace treaty of that year. During his absence from Connacht the confederates, including his uncles Sir James Dillon and Father George Dillon, managed to gain Athlone castle by subterfuge. On his return to Athlone, he was unable to retake the castle and although he intended returning to Dublin to vindicate his honour, his family and friends persuaded him to go to Kilkenny and join the confederates. There he took the confederate oath of association and was reconciled to catholicism. He sought the restoration of Athlone castle and Rinuccini (qv), believing Dillon's conversion to be sincere, ordered Captain MacGawley to give up the castle. However, the captain refused to obey the order, even when declared a traitor.
In 1647, despite his lack of military experience, General Thomas Preston (qv) appointed Dillon commander of the Leinster cavalry. This probably contributed to the poor performance of the confederate cavalry at the battle of Dungan's Hill in August 1647, when they fled the battlefield. In September 1647 he was mentioned in relation to the plan to ship confederate soldiers to France, a plan in which his uncle Sir James Dillon was also involved. After the 1648 cessation, he commanded the Leinster cavalry in a successful expedition to retake Athlone, although he was unable to prevent the army of O'Neill (qv) from wasting Co. Westmeath. He signed the 1649 peace treaty and was appointed to the ‘commission of trust’ responsible for administering those parts of the country still under catholic control. In August 1649 he was in charge of the 2,500 men who remained at Finglas while Ormond took the rest of the royalist army to Rathmines. In 1651 he attended the council of Clanricarde (qv) in Galway and was governor of Athlone. After the surrender of the Athlone garrison to parliament, many within the royalist camp did not trust him and he was replaced as an army general.
On 12 May 1652 his cavalry force was designated to surrender to the Cromwellians in Mullingar. The parliamentary commissioners declared that he should have no immunity and his estates in Mayo, Roscommon and Westmeath were seized. Dillon subsequently went into exile abroad, but he returned to Ireland shortly after the restoration of the monarchy in 1660. In May 1660 he wrote to Ormond, outlining his fears that the affairs of those who had loyally served the crown might be prejudiced by the many private applications to the king for satisfaction. On 16 March 1661 Charles II ordered that Dillon's lands were to be restored and his military arrears paid. Although still nominally joint lord president of Connacht, he resigned from the position in 1662, for which he was financially recompensed and appointed custos rotulorum for Co. Westmeath. He made his will in May 1673 and died later that year on in 1674.
Before 1636 he married Frances (d. 1674), daughter of Sir Nicholas White of Leixlip and Ursula, daughter of Garret, 1st Viscount Moore (qv) of Drogheda. The couple had at least three sons, two of whom, Charles (b. 1636), an army general in France and Spain, and Rupert, a page to Charles II, appear to have predeceased their father. The only surviving son, Thomas, who married Elizabeth, daughter of Sir John Burke of Co. Galway, briefly became the 5th Viscount Dillon of Costello-Gallen, though he died shortly after his father and the title passed to his cousin Lucas Dillon in 1674.