MacDonnell (MacDonald), Ustian (Austin, Justin) (d. 1588), rebel, was the son of Maolmhuire MacDonald. He became the leader of the somewhat obscure Clann Aedh Buidhe sept of Clandonnell (MacDonald, MacDonnell), gallowglass of Mayo, who were based in the barony of Kilmaine. He was described as the leading Clandonnell member of his time. In 1572 he mustered his forces in support of the rebellion of Ulick Burke (qv) and John Burke (qv), the sons of Richard Burke (qv), 2nd earl of Clanricard, during which the towns of Athlone and Athenry were burnt; in these campaigns he fought alongside the famous Munster rebel James fitz Maurice Fitzgerald (qv). The uprising forced the crown forces to withdraw from Connacht for four years. In 1576 MacDonnell was probably one of the seven Clandonnell leaders who submitted to the lord deputy, Sir Henry Sidney (qv), at Galway during his tour of the country. However, John and Ulick Burke led another rebellion later that year which many Clandonnell gallowglass, perhaps including Ustian, supported.
In July 1579 fitz Maurice, who had been in exile in Europe, returned to Ireland at the head of a small expedition sponsored by the pope, and wrote to Ustian for help. Perhaps motivated by this call, MacDonnell participated in the rebellion of Richard Burke (qv), 18th lord of MacWilliam Íochtar, in 1579–80. However, Sir Nicholas Malby (qv), governor of Connacht, quickly forced the submission of the rebel forces, including MacDonnell, who submitted on 12 February 1580 at Liskillen, Co. Mayo; he was pardoned in March 1584. Malby's incursion deep into Mayo boded ill for MacDonnell. Earlier he had been able repeatedly to defy the crown with relative impunity because the government had no influence in his Mayo base, but by the mid-1580s, having finally subdued south Connacht, the crown was expanding northwards into Mayo and began to threaten him directly for the first time.
The government's decision in summer 1586 to abolish the Gaelic title of MacWilliam Íochtar and thereby impose English patterns of land ownership in Mayo sparked a rebellion in the county to which MacDonnell gave his support. However, the scorched earth tactics of Sir Richard Bingham (qv), president of Connacht, quickly broke the rebellion, and a gaunt, half-starved MacDonnell submitted to Bingham in August, handing over one of his sons as a pledge for his future good behaviour. In September he was present at the battle of Ardnaree, Co. Sligo, at which an estimated 1,500–2,000 Scots gallowglass were killed by Bingham's forces. MacDonnell was pardoned again on 23 November 1587.
In 1588 Bingham arrested MacDonnell on the grounds that he was fomenting rebellion, providing aid to Spaniards washed ashore from shipwrecks of the Spanish armada, and actively preventing the collection of the composition of Connacht; he was executed under martial law at Donamona castle. The Gaelic Irish claimed that MacDonnell had voluntarily met Bingham to complain about the behaviour of the royal troops, and that he had been killed while under protection, which would constitute a clear breach of faith on Bingham's part. The ill feeling caused by his execution contributed to the outbreak of further rebellion the following year.