Thomas Dillon, 4th Viscount Dillon PC (Ire) (March 1615 – 1673) held his title for 42 years that saw Strafford's administration, the Irish Rebellion of 1641, the Irish Confederate Wars and the Cromwellian Conquest of Ireland.

As he was 15 at the time, he became a ward and the estate was seized by the King, who sold the wardship to Thomas's uncle Lucas Dillon of Loughglynn.

[34] About 1636 Wentworth's sister Elizabeth married James Dillon, 3rd Earl of Roscommon, a first cousin on his mother's side.

The purpose of the parliament was to raise subsidies for an Irish army of 9000[39] for Charles I to fight the Scots in the Bishops' Wars.

[41][42] On 3 April 1640 Strafford left Ireland,[43] called elsewhere by the King, having appointed Christopher Wandesford as Lord Deputy.

[47] The Irish MPs regretted having voted for subsidies and wanted to change how they would be evaluated and collected.

Dillon together with Gormanston, Kilmallock and Muskerry were sent to London by the House of Lords to report grievances.

[59] With the arrival of the papal nuncio, Giovanni Battista Rinuccini, in Ireland on 21 October 1645[60] Lord Dillon's Protestant religion caused him problems.

[61] He had left Athlone under the command of Captain MacGawly, who betrayed him and handed the town over to Owen Roe O'Neill.

[62] In August 1647, the Confederate Leinster army under Thomas Preston was severely beaten in the Battle of Dungan's Hill by Parliamentarian troops under Michael Jones.

On this occasion Lord Dillon commanded the Confederate cavalry, which fled in the early stages of the battle.

[71] In 1650 Lord Dillon successfully defended Athlone against a Parliamentarian army under Henry Ireton, skilfully holding him off by protracted parleys until Ireton decided to leave and rather reinforce Hardress Waller at the siege of Limerick.

[73] Lord Dillon's estates were confiscated by the Cromwellian Settlement of 1652, and he and his family lived in exile on the continent until the Restoration.

[75] In 1663 most of his extensive lands were restored by the Act of Settlement 1662, and several high offices in the state were conferred upon him, including that of Custos Rotulorum of Westmeath.

However, Dubhaltach Caoch Mac Coisdealbhaigh, who was descended from a family of the Gaelic nobility of Ireland that had been cheated out of their land by the 1st Viscount, resisted by organising rapparee actions.

[81] Most likely, Frances, his wife, outlived him and died in 1674, being buried in St. Mary's Chapel in Christ Church Cathedral.