When young he left Kinsale at its surrender in 1602 for Spain where he took service, first as marine in the Atlantic Fleet and then in the Army of Flanders.

He was confirmed as General of the Munster Army by the Irish Catholic Confederation but was in practice superseded by Castlehaven in 1643.

[9] Charles Blount, 8th Baron Mountjoy, Deputy of Ireland, rushed down to Munster and started the siege early in October.

[2][12] Barry served for four years as a marine in the Spanish Atlantic Fleet in the Irish company of Hugh Mostyn.

[21] In October 1641 Phelim O'Neill launched the Irish Rebellion from the northern province of Ulster.

[23] In January 1642 Barry and Maurice Roche, 8th Viscount Fermoy besieged Richard Boyle, 1st Earl of Cork in Youghal.

[27] In March 1642 Barry, Muskerry besieged William St Leger, the President of Munster, in Cork but were driven off by Murrough O'Brien, 6th Baron Inchiquin, on 13 April.

He then tried to invade County Cork again but was driven off in disorder at the Battle of Liscarroll on 3 September 1642 by Inchiquin with a numerically much inferior force.

When the insurgents organised themselves as the Irish Catholic Confederates in October 1642, Barry was confirmed as commander of the Munster army by the general assembly.

[30] In 1643 Barry prepared to besiege the town of Cappoquin in County Waterford, held by Inchiquin.

The Discourse was meant as an introduction to the subject for young Irishmen envisaging to take Spanish service as infantry officers.