Callaghan was destined for a Catholic religious career and entered a seminary in France where his family was in exile during Cromwell's rule.

His father was a member of the MacCarthy of Muskerry dynasty, a Gaelic Irish family descended from the kings of Desmond.

While Callaghan was a child, his father, Lord Muskerry, in March 1642 joined the Confederates[17] to defend the Catholic faith and, as he thought, the King.

In October 1645 Giovanni Battista Rinuccini, the papal nuncio arrived in Ireland and visited Macroom Castle where Callaghan and his family were living.

He fought to the bitter end and surrendered Ross Castle near Killarney to Edmund Ludlow on 27 June 1652, disbanding his 5000-strong army.

[26] The family's estates were lost in the Act of Settlement of 1652,[27] passed by the English Rump Parliament on 12 August.

[30] On 27 November 1658 his father was created Earl of Clancarty by Charles II in Brussels, where the King was then in exile.

Cormac stayed behind in Dunkirk with his regiment, whereas Callaghan continued to prepare for the priesthood in a French seminary,[33][34][c] In 1662 his father, Earl Clancarty, recovered his estates in the Act of Settlement.

[37] His brother had an infant son, also named Charles, who succeeded him as heir apparent and Viscount of Muskerry.

Callaghan and Elizabeth had one son: —and four daughters of whom three are known by name:[45] Clancarty died of a stroke ("Apoplexy") on 21 November 1676 in Dublin, aged about 38.