Thomas Eustace of Kerdiffstown, County Kildare (also called Cardeston) was the son of Nicolas Eustace and Anne Eustace. She was the daughter of the 1 ^st Viscount Baltinglass (Thomas Eustace of Harristown, and his wife Margaret Talbot). Thomas married Joan Eustace (daughter of William Eustace (of Moone-High Sheriff in 1544) and Margaret FitzGerald) of Mainham. William of Moone died 10 March 1557 age about 40. He was the son of Alexander Eustace of Clongoweswood and Joan, daughter of Christopher Eustace of “Kerdeston.”

Thomas Eustace joined the Baltinglass rebellion and in 1581 was executed together with his eldest son Christopher Eustace who was only nineteen and died with very great bravery. The estate at this time included Gaganstown, Swordlestown, Morganstown, considerable lands in Clane and lands in the parishes of Abbeyshrule, Taghsynnye, and Agherye in County Longford as well as parcels in the abbey of Loughsoudie and near Mullingar, County Westmeath. The forfeited estates, which were substantial, including Gaganstown, Swordlestown, Morganstown  and Clane were unsuccessfully claimed by Sir Maurice Eustace of Harristown in 1700.

Nicolas Eustace, father of Thomas, was the son of Waldron Eustace. Nicolas is listed as a juror in 1551. He married first Margery, daughter of Sir William Wogan, and second Anne Eustace, daughter of 1st Viscount Baltinglass. With his marriage to Anne Eustace about 1538, Nicolas acquired Gaganstown located immediately south of Coughlanstown as a dowry.

Waldron Eustace  married Margaret, daughter of Lawrence Sutton of Tipper. Waldron and Margaret were the parents of Nicholas above. Waldron was the son of Christopher of Kerdiffstown, who follows;

Christopher Eustace of Kerdiffstown, (father of Waldron and grandfather of Nicholas, and great-grandfather of Thomas), described as late of Kiltussy, was High Sheriff in 1465, 1474 and 1475. He leased Donode castle and its lands from Sir Robert Eustace of Ballycutland (Coghlanstown). By his wife Elizabeth Ashe of Forenaghts, Christopher had a son  Younger children of several of the above generations had married into other branches of the Kerdiff family.