The fierce struggle of O’Sullivan Mór against the forces of Cromwell comes as something of a surprise given the surrender and regrant arrangement with the crown entered into by the family two generations previously in the reign of James I. Their hostility to the English continued after the restoration of Charles II to such an extent that they receive mention in correspondence of Orrery with Ormond after Orrery failed to secure the capture of O’Sullivan Mór (Donal), and his two uncles whom he describes as French officers; all were part of the Irish resistance to the parliamentary commander Inchiquin in the year Inchiquin committed the terrible sack of the Rock of Cashel: 1647.
The Parliamentary forces held the upper hand in the English civil war from the second half of the 1640s in consequence of victories like Marsden Moor and Naseby, and the military genius of Oliver Cromwell. The principal prosecutor of the war against the Irish had important blood connections with the Irish, including those in Kerry, which may have served to temper his enthusiasm for victory. This was Murrough O’Brien, future Lord Inchiquin, to whom leadership of the Parliamentary prosecution of the war in Munster devolved in the years subsequent to the victories of Marsden Moor and Naseby. With the trend of events in his favour, and bringing with him his continental experience in the Thirty Years War in Germany, Inchiquin inflicted a heavy defeat on the Irish under Lord Taaffe on November 13, 1647 at Knocknanuss. Owen O’Sullivan Mór and his two uncles (the uncles who would enter the French service) were among the defeated Irish. The O’Sullivans retreated to the rocks and fastnesses of Iveragh, Dunkerron and Glanerought where they continued to wage war. Inchiquin was a first-cousin of O’Sullivan, their mothers being daughters of FitzGerald of Cloyne. This information is to be found in the genealogies of Black Jack Blennerhassett, as well as Lodge and Archdall’s Peerage of Ireland (1789). Inchiquin is said to have been fostered with FitzMaurice of Lixnaw. This was Patrick FitzMaurice, nineteenth Lord Kerry, whose wife was yet another daughter of FitzGerald of Cloyne. Inchiquin, according to the historian Jeremiah King, made efforts to reach out to his cousin O’Sullivan Mór, to no avail, and when the English under Vauclier took to the sea around the coast of South Kerry in an effort to secure his defeat, O’Sullivan inflicted a defeat on them when they landed. Vauclier’s name is also known for being the author of a number of depositions about the damage done to his property during the insurrection of the early 1640s. Inchiquin changed sides to the royalist Ormond when he saw the influence of the extremists on the parliamentarians, and he must have been reassured when the Pope’s nuncio, who had divided the Irish and fanned the flames of extremism, departed the country.
Lord Herbert and others confirmed the disturbed conditions in South Kerry on 27 May 1673, implicating “the three chiefs of the Sullivans themselves, namely O’ Sullivan More, O’Sullivan-Beare, and M’Gillycuddihy, although neither of them were adjudged innocent nor have any benefit of the late Art of Settlement, do nevertheless, viis et modis, enjoy considerable parts of their late estates …” (letter published in the Kerry Magazine, January 1856). However, their criticisms seem misplaced, at least where MacGillycuddy is concerned, for Donough MacGillycuddy was recently restored by Charles II to his Kerry estates, very probably through the influence of Ormond who was a far greater influence than the absentee Herbert of Chirbury.