Sir Thomas Eustace was born about 1480, and succeeded to his uncle's estates when only sixteen. He was the son of Richard Eustace and Anne Eustace.¹ He married Margaret Talbot, daughter of Sir Peter Talbot and Catharine FitzGerald.¹ He died on 31 July 1549.¹

On 14 December 1496 he succeeded his uncle, Rowland Eustace, Lord Portlester, in the family estates.¹ He was created 1st Baron Kilcullen, co. Kildare [Ireland] in September 1535.¹ He is said to have possessed one half of County Wicklow.¹ He was created 1st Viscount Baltinglass, co. Wicklow [Ireland] on 29 June 1541.¹ He lived at Harristown, County Kildare,.¹

Except that he was High Sheriff of the County in 1523 and was knighted sometime between 1528 .and 1533, we know nothing of him until the rebellion, in 1534, of his kinsman, " Silken " Thomas, 10th Earl of Kildare. His father, the 9th Earl (Portlester's grandson), before leaving on a visit to England (where he died, but was thought to have been murdered, in the Tower), had left instructions that his son, then still a minor, should be guided by the advice of the Earl of Desmond, the -Lord Chancellor (Archbishop of Armagh) and Sir Thomas Eustace. In defiance of their advice he raised forces against the government to avenge his father"s death, and one of his first acts was to seize Sir Thomas's Castle of Portlester. After other successes he besieged Dublin Castle and killed the Archbishop. The Eustaces (not for the last time) were sharply divided, and, among others of the family, Christopher of Ballycotland joined the rebels. Sir Thomas, however, collected forty of his relatives to aid the King with what retainers they could muster. In July, he and his men were present at the defeat of a much superior force of rebels engaged in the -interception of a. convoy on its way to Rathangan Castle, and the next month took a prominent part in the Battle of Allen. It was here that an elaborate plan had been made to surround the rebels, and as aresult many of them, including the Silken Thomas, were captured, not a few falling into the hands of Sir Thomas. Such however was the half-hearted ness of many of the government adherents, that they were " let goo agayne."

For his services, Sir Thomas was created, in 1535 Baron of Kilcullen. In 1537, he was appointed Constable of the Castle of Kilkea, and, with James FitzGerald of Osberstown (His wife was Margaret Eustace, sister of the rebel, Christopher Eustace of Ballycotelan, hanged in 1537), was made responsible for Lea Castle. For further actions against Irish rebels, he was advanced to Viscount Baltinglass in 1541, and was granted the very large possessions of the Abbey, although it is now difficult to identify the lands of Baltinglass, owing to changes in names and spelling. It was said that he held more than half of County Wicklow, in addition to his estates in Kildare and Meath. As one of the Lords of Parliament, his name appears that year in the historic Bill proclaiming Henry VIII King of Ireland. In 1542 he was chosen to take custody of the hostage held- to ensure the good behaviour of Rory O'More, when appointed Chief of Leix. In 1546, on the suppression of St. Patrick's, Dublin, he was granted the demesne and parish of Kilberry (see Athy). In 1549 he died at New Abbey, disestablished ten years before and in ruins except for a few rooms probably used for tending the sick. He had been granted the lease of the Abbey and grounds, still held by his son eleven years later.

He married Margaret, daughter of Sir Peter Talbot of Malahide Castle, Co. Dublin, by whom he had four son's and four daughters : ROLAND, the 2nd Viscount Richard of Boleybeg and later of Tullaghgorey, near Athy Alexander of Colbinstown (q.v.), who married Jenet, daughter of Robert Eustace of Oldcastle ; Robert of Tullaghgorey (see Athy), who (or perhaps his brother Richard) married in about 1542 an Irish girl named Dorothy O'More ; John who was High Sheriff of Kildare in 1535 ; Anne who married (1) one of the O'Tooles of Imail, Co. Wicklow, and (2) in about 1530 Nicholas Eustace of Kerdiffstown (q.v.) ; Janet who married (1) Gerald Sutton of Castletown, and (2) Maurice FitzGerald of Osberstown ; Margaret who married George Burnell ; and Catherine who married (1) James FitzGerald of Ballyshannon (or Carberry?) and (2) Gerald, son of Robert Plunkett, 5th Baron of Dunsany.