Dillon, Sir Lucas (1579–1656), MP and confederate, was the second son of Theobald, 1st Viscount Dillon (qv) of Costello-Gallen (d. 1624/5) and Eleanor (d. 1638), daughter of Sir Edward Tuite of Tuitestown, Co. Westmeath, and widow of William Tuite of Monilea. He was the elder brother of Sir James Dillon (qv) and uncle of Thomas Dillon (qv), 4th Viscount Dillon of Costello-Gallen. 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. He was knighted by the earl of Essex (qv) in 1599 and created Viscount Dillon of Costello-Gallen in March 1622. Lucas's main residence was at Lough Glynn in Co. Roscommon, though he also held land in counties Mayo, Louth, Sligo and Westmeath, and he was widely connected, by marriage, to many prominent catholic families.
Dillon, who appears to have had some legal training, sat on the commission of boundaries set up to inquire into the estates of the earl of Tyrone (qv) in 1607. He was knighted in 1621 and served as a member of the commission of peace in 1624/5. He was one of the signatories to a petition from the lords of the Pale to Charles I in 1625 and, in July 1627, he was listed among those appointed to raise funds for the army in Co. Mayo. In early 1628 Dillon, recommended to the privy council for his competency and loyalty, was one of two Connacht members of the Irish delegation that travelled to London to negotiate with Charles I; this negotiation resulted in the granting of the ‘Graces’ the following May. He was appointed a commissioner for fees and exactions by the king in May 1628 and he was allocated one of the new military companies, which were to be raised in Ireland. During the summer of 1629, he sought the wardship of his grand-nephew the infant Theobald, 3rd Viscount Dillon of Costello-Gallen, to ensure that he was raised a catholic and the family estates protected. Together with Lord Wilmot (qv), he was granted the wardship in November 1629, but the infant died in 1630 and the title reverted to Dillon's nephew, Thomas, 4th Viscount Dillon.
Dillon was elected as MP for Roscommon in 1634 and again in 1640. His position during the early stages of the rebellion of 1641 remains somewhat uncertain, though certain accounts suggest that he joined the rebels. He was one of the Connacht gentry who took an oath to uphold the catholic religion and the royal prerogative. In February 1642 he brought a letter from members of the gentry in Roscommon to Clanricarde (qv), explaining why they had joined the rising and seeking his support. In October 1642 he joined the confederate association, where he sat in the general assembly and on the supreme council as representative for Connacht. Dillon was among the group of confederates who entered into negotiations with representatives of the king at Trim in March 1643. He actively negotiated the 1643 cessation and was one of its signatories on 15 September 1643. As an agent for Connacht, Dillon met the marquess of Ormond (qv) at Kilcullen in June 1644. Although it was reported that he was shot during fighting in Co. Sligo in October 1645, he was still commanding his troops in Co. Mayo in August 1646.
Closely associated with the peace faction on the supreme council, Dillon was actively engaged in peace negotiations with Ormond during 1645–6. He was suggested as a possible privy councillor in March 1646 and, four months later, he signed the first Ormond peace treaty. Following the successful clerical coup of September 1646, he was imprisoned, along with other supporters of the peace, and was excluded from the supreme council between September 1646 and November 1647. He was sent to the Ulster army of Owen Roe O'Neill (qv) in April 1647, to administer a new oath of association and to distribute money. Rinuccini (qv) was unsuccessful in his attempt to block Dillon's reelection to the supreme council in November 1647. In 1648 he supported the truce with Inchiquin and appealed to the pope against the nuncio's excommunication. He was one of the confederate commissioners appointed to negotiate the second peace treaty with Ormond in 1648–9.
Once the peace was finally concluded in January 1649, Ormond appointed Dillon as vice treasurer and, when the supreme council was replaced by a ‘commission of trust’, he was one of twelve commissioners delegated to administer those parts of the country still under catholic control. Throughout 1650–51 he endeavoured to maintain the royalist cause and in 1651 he was among those summoned to Galway by Clanricarde to discuss the declining royalist position. After the final surrender, Dillon's estates were sequestered and he was imprisoned, for a time, in the Marshalsea in Dublin. He died in 1656 and his widow and family were transported to Co. Westmeath. His wife was Jane Moore of Brees, Co. Mayo, and the couple had four sons.