In 1520 he defeated James FitzGerald, 10th Earl of Desmond in the battle of Mourne Abbey.

MacCarthy married Catherine, daughter of John Barry, 1st Viscount Buttevant.

In 1498 MacCarthy, with help from Thomas FitzThomas FitzGerald, future 11th Earl of Desmond, killed Owen.

In September 1520 Muskerry and Donal MacCarthy Reagh helped Thomas FitzGerald, 11th Earl of Desmond, defeat James FitzMaurice FitzGerald, 10th Earl of Desmond in the battle of Mourne.

In December, together with Piers Butler, 8th Earl of Ormond, they besieged James FitzMaurice FitzGerald, in Dungarvan.