MacCarthy, Callaghan (d. 1676), 3rd earl of Clancarty , nobleman, was born in Ireland in the late 1630s, the son of Donough MacCarthy (qv) (1594–1665), 1st earl of Clancarty, of Macroom, Co. Cork, and Eleanor Butler, sister of James (qv), 1st duke of Ormond. He was the second of three sons, of whom the eldest, Charles MacCarthy (1633–65), Viscount Muskerry, who died in action against the Dutch in the naval battle at Solebay on 2 June 1665, and the youngest, Colonel Justin MacCarthy (qv) (1642–94), Viscount Mountcashel, were close associates of the duke of York, later James II (qv). There was at least one sister, Margaret, who married Luke Plunket (qv), 3rd earl of Fingall.
Although he was descended from the most politically powerful Gaelic Irish family in Munster, Callaghan's father aligned himself with Ormond and those who supported the peace treaties in the confederation of Kilkenny in the 1640s, in defiance of the papal nuncio GianBattista Rinuccini (qv). His family joined the Stuart court in exile and MacCarthy spent several years with his mother at the convent at Port Royal, France. He entered a seminary in the late 1650s, where he adopted the family's Jansenist tradition, and by 1664 he was a student at the Irish college at Toulouse.
Justin had been expected to succeed to the earldom but Callaghan abandoned his studies and returned to London in September 1666 to claim his right. He inherited lands which had been restored to his father by personal proviso in the acts of settlement (1662) and explanation (1665). The estate had been considerably augmented by the inclusion of the lands of certain freeholders in Muskerry. The old earl had undertaken to acquire these lands on their behalf and later return them or lease them on easy terms. Callaghan, however, did not feel obliged to honour this arrangement and considered the estate too indebted to bear the loss of any assets. The title to the estate was, in fact, found to be defective and matters had to be quickly and secretly rectified. To add to the earl's financial problems Anne, marchioness of Clanricard, sued for payment of a bond for £8,000 posted by his late father and brother Charles for the rent of her estate at Somerhill, Kent. Callaghan also quarrelled with Justin over the latter's demand to have lands in lieu of the £1,000 left to him in their father's will. Ormond relieved the situation by ensuring that a small estate at Ovens, Muskerry, was settled on Justin, but the relationship between the brothers remained hostile. The dispute with the freeholders, despite extended litigation and mediation, was settled only on 17 October 1673, when the Irish privy council ordered Callaghan to grant ninety-nine year leases to his tenants, who would in turn pay a share of the quit rents. A court was established at Macroom specifically for this purpose. Despite a reduction in quit rents, negotiated with the lord treasurer in 1675, and the earl's constant efforts, the estate remained encumbered by debt. His plans to leave each of his daughters £2,000 could not in the end be realised. In the 1660s the earl was politically aligned with his uncle, the duke of Ormond, and had no public involvement with the representations made by Colonel Richard Talbot (qv) for a review of the Irish land settlement in the early 1670s.
In 1667 Callaghan married Elizabeth, daughter of George Fitzgerald (qv), 16th earl of Kildare, and Joan, daughter of Richard Boyle (qv), 1st earl of Cork; they had one son, Donough MacCarthy (qv), and four daughters. The marriage agreement was negotiated by Ormond, the earl's uncle, and Elizabeth's uncle and guardian Roger Boyle (qv), earl of Orrery. Religious difficulties expressed by both parties were apparently resolved and Callaghan subsequently conformed to the Church of Ireland. When the earl died suddenly from apoplexy in Dublin in 1676, however, he was described as being outside the established church. Despite this he was buried at Christ Church cathedral on the evening of 21 November 1676.