He came from an old landed Hiberno-Norman family in County Cork in southern Ireland. Like many Irish Catholic gentlemen of his generation, particularly younger sons, he left Ireland to pursue a military career in Catholic Europe. He served for almost thirty years in the Irish regiment of the Spanish Army of Flanders against the forces of the United Provinces of the Netherlands.
He returned to Ireland in 1640, hoping to recruit men for the Spanish Army. However his enterprise was interrupted by the outbreak of the Irish Rebellion of 1641 - an uprising of Irish Catholics against English Protestant rule. Barry sided with his fellow Catholics, organised in the Confederate Catholic Association of Ireland, who made him general of their forces in Munster. Barry took Limerick for the Confederates through skillful use of the siege techniques he had learnt in Flanders - in particular under-mining of the walls of the castle there (see Siege of Limerick 1642). However, Barry failed to take Cork and was driven off in disorder at the battle of Liscarroll when his forces advanced on the city. Discredited, Barry took little further part in the war and died in Limerick in 1647.
Barry was also a prolific author on military matters, writing several handbooks on how to train and discipline troop and how to handle them in battle.