Butler, Thomas (d. 1419), prior of Kilmainham and deputy lieutenant of Ireland, was the illegitimate son of James Butler (qv), 3rd earl of Ormond, and an unknown woman. He rose to the position of prior of the Hospitallers of Kilmainham, seemingly in his early 20s – probably due to the influence of his father. He may not have been destined for religious orders, as he is mentioned as the subject of a marriage contract (January 1401), but the negotiations must have collapsed, as he was elected prior in 1403. His election was contested by the English order as part of their campaign (supported by Henry IV) to control the Irish order; but he was eventually recognised in 1407, and confirmed in a general chapter of the order in 1410.
As prior of Kilmainham, Butler was active in the defence of the lordship, especially the earldom of Ormond during the minority of his nephew, and was appointed deputy lieutenant to Thomas of Lancaster (qv) before March 1409. His four years in this post were not without problems, and he twice refused to obey summonses from the king. His period in office officially ended with the death of Henry IV (April 1413), but he continued to act as justiciar till the arrival of John Stanley (qv) in September 1413.
Butler was an important figure in the opening phase of the Talbot–Ormond feud, as he forcefully defended the Ormond lordship against the new lieutenant, John Talbot (qv), Lord Furnivall. Perhaps in an attempt to cool the feud, Henry V summoned Butler to service in his campaigns, where his group of Gaelic and Anglo-Irish troops were regarded as a strange novelty. He served at the siege of Rouen, dying there in 1419, leaving his lands and manors to his sons, who formed one of the minor Butler clans later in the century.