Callaghan (MacCallaghan), John (1605–64), Jansenist priest and agent, was born in Killone (Carbury), near Macroom, Co. Cork, fifth and youngest son of Dermot MacCallaghan and his wife Catherine (née MacCallaghan), a distant cousin. Raised in impoverished conditions, he dropped the ‘Mac’ from his name early in his life. Educated in classics in Ireland, he moved to France to become a priest. There is some controversy over where he was trained: Callaghan himself said that he was with the Oratorians at Nantes (1626–8), and then at Rennes (1629–30). The Jesuit priest Brisicier, however, later alleged that Callaghan had actually been a cleaner and corrector – while also studying – at the Jesuit school at Quimper (1626–7), and produced much evidence to substantiate this claim. Callaghan vehemently denied the accusation, and the whole controversy is seen as a test case for the questions surrounding his integrity. In 1630 he became an extern student at the Jesuit college of La Flèche. He completed his education at the Sorbonne, where he received his doctorate in 1642; although the validity of this has been questioned, it appears to have been legitimate. It seems that he became a Jansenist at the Sorbonne, under the influence of Abbé Mazure, the curé of St Paul.
After completing his doctorate Callaghan returned to Ireland, where his movements attracted the suspicion of the Irish Jesuits, who reported him to Luke Wadding (qv), necessitating his swift return to France. He returned to Ireland in 1646 as part of a secret Jansenist mission. When the supreme council of the confederate catholics recommended him for the see of Cork, their proposal was vetoed by the papal nuncio Rinuccini (qv). Callaghan's links with Lord Muskerry (qv) and the marquess of Ormond (qv) increased the nuncio's mistrust. Nevertheless, in 1647 Rinuccini asked him to preach in Kilkenny, showing perhaps some measure of respect. Shortly after this, Callaghan returned to France. He was friends with Antoine Arnauld, a Jansenist apologist, and his sister Angélique, the abbess of Port Royal. With her support he founded an Irish house in the Faubourg Saint-Marceau, Paris. In 1648 he supported the Carmelite friar John Rowe against Rinuccini in the controversy over the nuncio's excommunication and censure of opponents in Ireland. Their appeal to the theological faculty of the university of Paris was unsuccessful, however, and reflected badly on Callaghan.
In 1650 Callaghan wrote an influential, and controversial, account of Ireland (1641–9) that he published, under the pseudonym of ‘Philopater Irenaeus’, as Vindiciarum catholicorum Hiberniae libri duo. This work, which has been wrongly attributed to Richard Bellings (qv) and which included a strong defence of the Inchiquin truce, refuted Epistola nobilis Hiberni . . . (1649), a defence of Rinuccini by Paul King (qv). Callaghan became parish priest of the village of Cour-Cheverney, near Blois in 1650. He immediately attempted to impose his Jansenist beliefs, but met with much resistance. The rector of the Jesuit college at Blois denounced him from the pulpit as an enemy of the catholic church, and a gateway to hell. This precipitated a vicious war of words, as Arnauld and others leaped to his defence. A satire in Latin, Calaghanus an satyrus? (Is Callaghan a satyr?), alleged that Callaghan came from a swinish background and had bartered his soul for his doctorate. In 1653 Callaghan urged the exiled Charles II to seek assistance from Rome, and went there himself as an envoy (April), with the power to promise freedom of religion in Ireland for catholics. This journey, however, was unsuccessful.
There were reports of Callaghan's death in 1654, but these were mistaken and he continued to live at Port Royal until 1664. It seems that he was buried in a rural convent of the Jansenist nuns, alongside St Cyran, a leading Jansenist, where their bones were venerated.