Magennis, Arthur (1623–84), 3rd Viscount Magennis of Iveagh , was the son of Hugh Magennis and his wife Mary Bellew of Castletown, Co. Louth. Though a minor at the time of his father's death in 1639, he was not made a government ward, as he was too old to be a likely convert to the Church of Ireland. His inheritance of 45,006 acres in the barony of Iveagh in south Co. Down was heavily indebted; though valued at £1,200 a year, a third of this was tied up in a jointure to his grandmother Lady Sarah O'Neill (daughter of Hugh O'Neill (qv), earl of Tyrone), and the rest encumbered by a debt of £4,500.
Magennis claimed to have been in Dublin as a royal ward when the 1641 rising broke out, and a witness later placed him at Duleek in December 1641, with the Irish army encircling the town of Drogheda. By February of the following year he had left the besiegers and moved north to Iveagh, where his close relatives were already leading the insurgents; these included his uncles Daniel and Art óg and his mother, condemned as ‘so cruel against the English’, while another witness described Magennis as ‘a young but a desperate & cruel rebel’ (Depositions, Co. Down, TCD MS 837, Capt. Henry Smith, f. 134/17; Elizabeth Croker, f. 4b). In April and May 1642 he led an army of some 2,500 men vainly trying to block the southward advance of Robert Monro (qv) and his newly arrived Scottish Covenanter army. Monro subsequently captured Newry, and Magennis lost control of the territory of Iveagh throughout the 1640s.
Magennis subscribed to the Confederate oath of association; having been driven out of Ulster, he was picked in July 1643 as a member of the supreme council, the standing executive of the Confederate Catholic regime. The following year the supreme council allowed the French to recruit an infantry regiment in Ireland in return for subsidies. Chosen as colonel of this regiment, Magennis sailed from Galway with an under-strength contingent of 300 men, apparently recruited in Leinster. He subsequently returned and served as colonel of an infantry regiment in the army of Owen Roe O'Neill (qv) during the latter's campaign in Connacht in the summer of 1647. By then, the Confederate Catholics were beginning to split into ‘Ormondist’ and ‘clericalist’ factions, divided mainly by their acceptance of religious guarantees in any proposed peace treaty with the royalists. O'Neill's Ulster army, too, split in May 1649. Those officers who owned land in Ulster before the war, including Magennis, Alexander MacDonnell (qv), and Phelim O'Neill (qv), sided with the Ormondist party. The lightning campaign by Oliver Cromwell (qv) in Ireland (September 1649–May 1650) forced an uneasy rapprochement, and in March 1650 Magennis formally acknowledged to the catholic primate that he had sinned, and in return the primate lifted the sentence of excommunication laid on him for his adherence to the Ormondist party. By then his regiment had formed part of the garrison defending Wexford when it was stormed by Cromwell in October 1649. Magennis remained fighting until the bitter end of the catholic and royalist cause. On March 1653 he signed articles for the surrender of Belturbet, Co. Cavan, making this – along with nearby Cloughoughter (surrendered in April) – the very last Irish strongholds to hold out. The conditions of surrender allowed him to recruit a regiment for service abroad, and he fought as a captain in the duke of Gloucester's regiment of infantry in Spanish Flanders.
Magennis lost all his estates in the Cromwellian confiscations, but his prospects of regaining them must have seemed favourable after the restoration. In a petition to Charles II he claimed preferential treatment because he was under age when the 1641 rising broke out, because he had been the last royalist colonel to surrender, and because he served Charles II abroad. Magennis headed the list of well over 200 officers or ‘ensignmen’ who served in the royalist regiments in exile. The act of settlement and explanation allowed ensignmen to recover their lands without having to prove their innocence, but only if the Cromwellian grantee was fully ‘reprised’ or compensated beforehand, receiving the exact number of acres elsewhere. At the end of 1662 Magennis complained that he was still landless and forced to wait for two years in London, where he had been thrown in debtor's prison. In April 1666 he was one of the surviving catholic nobles and gentlemen who subscribed to the ‘humble remonstrance’, an abject assertion of loyalty and a disclaimer of ‘all foreign power’. It did him little good. In May 1666 the claim of Sir George Rawdon (qv) to lands he was disputing with Magennis was accepted by the commissioners, so Magennis was ultimately (September 1668) reduced to petitioning for left-over lands anywhere. Instead the king awarded him an annual pension of £150, subsequently raised to £500. This was £1,600 in arrears at the time of his death in 1684.