Clare, Thomas de (c.1245–1287), magnate, was the second son of Richard de Clare, earl of Gloucester and of Hereford, and Maud, daughter of John de Lacy, earl of Lincoln. He attended Oxford university with his younger brother Bogo from 1257 to 1259. Initially destined for a clerical career, he quickly became involved in politics through the activities of his brother, Earl Gilbert de Clare. He sided with Simon de Montfort and was knighted by him before the battle of Lewes (14 May 1264). After the baronial victory he formed a lasting friendship with the Lord Edward during his captivity, and after his brother broke with de Montfort aided his escape. He fought at Evesham, gained the king's pardon, and engaged in the seizure of a number of baronial estates, quickly becoming wealthy. He took the cross with Edward in 1267 and accompanied him to the East, returning to England with four saracen captives in February 1272. Between April 1272 and January 1273 he was in Gascony acting on behalf of Edward and King Henry. His association with Edward led to his appointment to lucrative positions including the stewardship of a number of royal forests. Although his interests in Ireland quickly absorbed much of his time, when he returned to England he was often employed in administrative tasks.
Thomas's connections with Ireland began in April 1269, when he was granted the wardship of the heir of Maurice FitzGerald (qv) (d. 1268), 3rd baron of Offaly, and he may have visited Ireland before he sold it to William de Valence for a sizeable sum. After Edward's return from the East in August 1274 and his coronation, Clare was sent to Ireland probably to support the new justiciar, Geoffrey de Geneville (qv), another close associate of the king. He arrived shortly after October and was appointed sheriff of Limerick, a position he held until June 1276. He briefly acted as chancellor in 1275 and was employed on various commissions for the king. Early in 1275 he married Juliana, daughter of Maurice fitz Maurice FitzGerald (qv), a former justiciar and important landholder in Munster and Connacht; through her Thomas gained a substantial landed interest in Limerick and Cork. An exchange of lands in England with Robert de Muscegros brought him the large lordship of Bunratty. This was followed on 26 January 1276 with an opportunistic grant by Edward to him of all of Thomond, to be held as a fief of the crown for the comparatively small service of ten knights.
After receiving the grant, Thomas commanded a large expedition against the Irish in Wicklow; if it was successful he had been promised by the king a supply of knights to help conquer Thomond. His army was severely defeated at Glenmalure and he was lucky to avoid capture. In 1277 he was part of another campaign in Wicklow, this time apparently successful. Soon afterwards he moved to Cork and began to aid the dispossessed Irish king, Brian Ruad O'Brien (qv). In an astonishing incident Thomas turned on his erstwhile ally and murdered him in cold blood while he was under his protection in Bunratty castle. Brian's son Donnchad O'Brien, despite his father's death, also allied himself with Clare. Meanwhile as desultory internecine warfare continued between the O'Brien factions, Clare strengthened his hold over parts of Thomond. In 1278 a petition to the king for aid probably resulted in a grant of the knights’ service of Ireland, which he used in laying the foundations of a large castle at Quin, near Ennis (1279). Thomas sat on the king's Irish council in 1280, advising the justiciar, Robert of Ufford (qv) (d. 1298), who with Theobald Butler (qv) (d. 1285) led a large expedition into Thomond, which put down resistance and allowed the completion of Quin (1281).
Relative peace ensued, which enabled Thomas in 1282 to renew his royal connections, acting as an agent in Ireland to raise loans for the king's Welsh expedition. His efforts were greatly appreciated by his patron, who commended his diligent service in Ireland and promised more royal favours. Clare went to England in November 1283 and spent a year in Edward's company as he travelled through Wales. He was forced to return to Thomond late in 1284, fearing that the murder of Donnchad O'Brien by his rival Toirrdelbach O'Brien (qv) might presage more devastation in Thomond. Some sort of accommodation was apparently reached between Thomas and Toirrdelbach; an inquisition after Thomas's death revealed the payment of a substantial rent by Toirrdelbach to the lord of Thomond for the possession of some seven cantreds and sixteen vills. Thomas resumed his activities on behalf of the crown in Ireland in 1285–6 and little more is heard of him until his death, apparently from natural causes, on 29 August 1287. He left a large and impressive endowment for his children, three sons (one illegitimate) and two daughters.
A descendant of Strongbow (qv), Thomas de Clare's Irish career bears many similarities to that of the original Anglo-Norman conquerors (except that he was much more consistently an agent of royal policy in Ireland). The traditional route for the landless second son of a wealthy and powerful comital family in England beckoned, but through a combination of resourcefulness and assiduous royal patronage he became an influential and wealthy lord in his own right. The speculative grant of Thomond meant nothing unless he was willing to campaign vigorously to ensure that his lordship was more than merely nominal. His ability to immerse himself in the murky world of O'Brien faction fighting and his ruthless opportunism did not, however, endear him to many of his Irish contemporaries. The Irish remonstrance, a list of complaints about the injustices of English rule in Ireland, sent to the pope in 1317, included his treacherous murder of Brian O'Brien. The Cathréim Thoirdhealbhaigh, a literary tract of the mid-fourteenth century celebrating the deeds of O'Brien, portrays Thomas as an archetypal villain and suggests that his many crimes were due to a vindictive capriciousness.