Eustace, Sir Thomas (1480–1549), of Harristown, Co. Kildare, 1st Viscount Baltinglass , was the only son of Richard Eustace, son of Sir Edward FitzEustace of Castlemartin, lord deputy (1452–4), and his wife Anne, daughter of Robert Eustace of Ballyloughrane. He succeeded to the estates of his uncle, Sir Roland FitzEustace (qv), Baron Portlester, when only 16. He became high sheriff of the county in 1523; and was knighted sometime between 1528 and 1533. Though he was not prominent in state affairs, he was clearly a valued supporter of his cousin, the 9th earl of Kildare (qv).
In February 1534 when Kildare, then lord deputy, obeyed a summons to court and left his son Thomas (qv) to deputize for him, Eustace was one of the five close associates on whose guidance he advised his son to rely. When Thomas raised forces against the government, it was contrary to the advice of Eustace, among many others. The slowness of the government’s reponse to the revolt made resistance to Thomas impracticable for some months, but Eustace took an early opportunity to oppose him. Shortly after the arrival of an English army led by the new Deputy, Sir William Skeffington (qv), in late October, he offered the services of himself and his kinsmen to the earl of Ossory (qv), who commanded the territory in Thomas's rear, and left hostages as an earnest of his sincerity. Although his action divided the Eustaces, and his cousin Christopher of Ballycotland was later executed, Sir Thomas remained in action and was with the government forces at the battle of Allen in August 1535.
In 1536 he was created baron of Kilcullen and in the following year he was appointed constable of the castle of Kilkea in Kildare and, with James FitzGerald of Osbertstown, was made responsible for Lea castle in Laois. On 30 March 1538 he was granted a small portion of the lands of the Cistercian monastery in Baltinglass. Finally, in 1541 he benefited from Lord Deputy St Leger's (qv) policy of granting crown properties to the loyal heads of powerful families along the borders of the Irish: the extensive possessions of the Baltinglass monastery were granted to him in tail male and he was created Viscount Baltinglass. In the same year, he attended the house of lords in parliament. In 1542 he was entrusted with the custody of the hostage taken to ensure the good behaviour of Rory O'More (qv) when he was appointed chief of Leix, and in 1546 he was granted the demesne and parish of Kilberry near Athy.
He died in 1549 at New Abbey, Kilcullen, which had been founded by Sir Roland Eustace (qv) for the Franciscans in 1486. Though it was suppressed in 1539, the friars had earlier given their goods to Eustace for safekeeping and, since it was one of the few abbeys to re-emerge in Mary's reign, it is probable that his patronage had enabled it to survive. Thomas Eustace married Margaret, daughter of Sir Peter Talbot of Malahide castle, Co. Dublin. They had five sons (Roland, Richard, Alexander, Robert, and John) and four daughters (Anne, Janet, Margaret, and Catherine).