Justin McCarthy, 1st Viscount Mountcashel, PC (Ire) (c. 1643 – 1694), was a Jacobite general in the Williamite War in Ireland and a personal friend of James II.

He commanded Irish Army troops during the conflict, enjoying initial success when he seized Bandon in County Cork in 1689.

Justin was two in 1645 when his mother hosted Giovanni Battista Rinuccini, sent as nuncio to Ireland by Pope Innocent X, at Macroom Castle.

[22][23][24] Around that time, anticipating the loss of Macroom or because of it, his father sent Justin, his mother, and sisters to security in France.

Muskerry fought to the bitter end, surrendering Ross Castle near Killarney to Edmund Ludlow in June 1652.

He became a professional soldier and showed great skill in his profession, but poor eyesight hampered his career.

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

Charles II decided to use his services in Ireland, and made him a colonel in Sir Thomas Dongan's regiment.

On the outbreak of the Popish Plot, however, the discovery of Colonel McCarthy's presence at Whitehall caused uproar: he fled the country, and the Secretary of State, Sir Joseph Williamson, who had issued his commission, was sent to the Tower of London.

The marriage was a failure, and Kenyon, Sunderland's biographer, remarked that it left a stain on the reputation of all those who ruined the lives of these two young people, without gaining anything in return.

[36] Under the Catholic King James II, McCarthy was in 1686 promoted to Major General and became a member of the Privy Council of Ireland.

[29] In 1688 or early in 1689, Tyrconnell appointed him Muster-Master General in the Irish Army and Lord Lieutenant of County Cork.

[38] On 23 May 1689, James II elevated Justin McCarthy to being Viscount Mountcashel, with the subsidiary title of Baron Castleinch.