The Irish Jacobite Army Book - Four Courts PressThe Irish Jacobite Army By Harman Murtagh & Diarmuid Murtagh. The Irish Jacobite Army was the largest body of Irish Soldiers ever to go into battle prior to the twentieth century. Although largely a new force, for three years, in alliance with France, it sustained a major war against a multi national and more professional Williamite army, involving pitched battles, sieges and other military operations throughout Ireland. S. R