Mandeville, Henry de (d. 1337), seneschal of the earldom of Ulster, was a leading member of a powerful family of vassals of the de Burgh earls of Ulster. Another Henry de Mandeville (d. 1272) had been seneschal in 1270, and Thomas de Mandeville occupied the office at the time that Edward Bruce (qv), claimant to the Irish throne, was in Ulster (1315–18). During Bruce's occupation of much of the eastern earldom many of the English of Ulster joined him, including, apparently, Henry's brother Richard de Mandeville, who was later charged with accompanying Bruce with ‘banners displayed in a warlike manner’. But after Bruce's death at Faughart in October 1318 the English of Ulster returned to their old allegiances.
Mandeville was seneschal of the earldom by 1323 and in the favour of Richard de Burgh (qv) (d. 1326), called the Red Earl. In that year de Burgh granted him ‘the intendance of all satellites of our bonaght in Ulster’, meaning that he had the right as seneschal to command the quotas of mercenary soldiers billeted on each of the Ulster chiefs by the earl. Mandeville enjoyed good fortune while de Burgh lived, but after his death Ulster came under threat from Robert Bruce (d. 1329), king of the Scots, who took advantage of the transition of power in the earldom to launch an invasion of Ulster. In spring or early summer 1327 Bruce landed at Larne, and Mandeville as seneschal decided to receive his envoys. This pragmatism culminated on 12 July in the sealing by Bruce and Mandeville at Glendun of a peace agreement to last a year. By its terms Mandeville agreed to deliver to the Scots at Larne 100 cendres of wheat and 100 cendres of barley. Half was to be delivered at Martinmas and the other half at Pentecost. However, the agreement did not find favour in all quarters, and a copy of it was sent to Edward II along with an alarming account of disorder throughout the earldom.
The new earl of Ulster, William de Burgh (qv) (d. 1333), known as the Brown Earl, had grave suspicions of the Mandevilles’ loyalty, fearing their power within the earldom and the strength of their connections with the English and Irish of Ulster, which the Mandeville brothers had enhanced through marriage. Henry married Elizabeth Fitzwarin, a member of a family who had formerly competed with the Mandevilles for the seneschalship, while Richard de Mandeville married Gyle de Burgh, the sister of Walter de Burgh (qv) of Mayo (d. 1332), who was William's enemy. Soon after his return from England with his wife in December 1330, Henry was implicated in the plot of Maurice fitz Thomas FitzGerald (qv) (d. 1356), 1st earl of Desmond, to make himself king of Ireland with the help of prominent English and Irish nobles. After a general rising, it was alleged, Desmond was to take the crown of Ireland and reestablish the old provincial kingdoms; Mandeville was to have Ulster. When news of the plot broke, de Burgh expelled Mandeville from Ulster, chasing him into Leinster, where, in September 1331, he was captured by Simon fitz Richard, a justice of the Dublin bench; he was conveyed to Dublin castle and imprisoned.
De Burgh now accused Mandeville of committing murders and robberies throughout Ulster in the first six months of 1331 and demanded that he stand trial in his liberty of Ulster, presumably as a demonstration of the strength of his rule. The government demurred and delayed a decision. In November the earl moved against the Mandevilles’ allies, arresting Walter de Burgh and imprisoning him at Northburgh castle, where he died during 1332. According to the annals of Friar John Clyn (qv), Gyle de Burgh and her husband, Richard de Mandeville, formed a plot to assassinate the earl, and on 6 June 1333 Richard's son Robert, with Robert fitz Martin de Mandeville and John Logan, murdered William de Burgh at Belfast, forcing his wife, Maud of Lancaster, and their infant daughter to flee by sea to England. Ulster rapidly descended into anarchy, as the Irish rose up to exploit the power vacuum. The justiciar succeeded in quelling the disturbances by July; the murderers fled to the Irish, while much of the earldom west of the Bann was lost forever. The government knew that they needed Henry de Mandeville to reestablish order in Ulster, but Maud of Lancaster grimly opposed his release on any grounds, and in November 1333 the English council declared that he should remain in captivity. However, the justiciar, John Darcy (qv), was determined to have him freed, so that he could negotiate with the nobles of Ulster and on 10 March 1334 he had his way.
Mandeville fulfilled Darcy's aim, using a combination of threats and rewards to bring the Ulstermen to peace. That summer he obtained permission to parley with the Irish in south Ulster, and in December he retook the castle of Greencastle, Co. Down, becoming its constable. He continued in 1335 to cooperate with the government, aiding Prior Roger Outlaw (qv) (d. 1341), deputy to Darcy, against the Irish of Ulster and campaigning for Edward III (d. 1384) in Scotland. His death in 1337 is recorded in the Annals of the Four Masters under his Irish name of Henry ‘MacMartin’.