Justin McCarthy, Viscount Mountcashel (died 1 July 1694) was a Jacobite general in the Williamite War in Ireland.
McCarthy was the younger son of Donough MacCarty, 1st Earl of Clancarty, from the MacCarthy of Muskerry dynasty who had lands in County Desmond. The family had had their property confiscated under the Cromwellian regime, but restored in the restoration. Justin McCarthy was made Viscount Mountcashel with the subsidiary title of Baron Castleinch on 1 May 1689 and became a Lieutenant-General.
With 3,000 men he advanced from Dublin towards Enniskillen, which with Derry was the remaining resistance to James II. He was met by 2,000 Protestant 'Inniskillingers' at the Battle of Newtownbutler on 31 July 1689. Mountcashel's forces were routed; he was wounded, then captured and finally exchanged while most of his men were put to the sword.[1]
He went into exile in France and commanded the first Irish Brigade of Louis XIV.