A new book sheds light on a long forgotten part of Irish history – the Irish Brigades, Catholic Ireland's overseas army.

Amateur historian Stephen McGarry,  the author of 'Irish Brigades Abroad,' told TheJournal.ie that he was inspired to write the book after reading John O’Callaghan’s 1870 book "History of the Irish Brigades in the Service of France."

“[O'Callaghan's] book fascinated me and I was surprised to learn that little else was written about the subject before or since,” he said.

He decided to update O'Callaghan's book, and the work, which he thought would take only one or two years, instead “became a long work of love which took me five years to complete, in between working full-time and raising a young family."

McGarry's work is now the first standalone history written in nearly 150 years on the topic and looks at the Irish regiments in France, Spain, Austria and elsewhere, covering period from King James II's reign in 1685 until the disbandment of the brigades in France and Spain.

"What emerges is a picture of the old-fashioned virtues of honor, chivalry, integrity and loyalty, of adventure and sacrifice in the name of a greater cause," says McGarry.

He says the Irish military diaspora in France and Spain has “more often than not, been too easily dismissed by historians as simply mercenaries lacking a political ideology.”

McGarry's book sets out to prove this isn’t true, noting that they “supported the deposed King James II as the Jacobite army-in-waiting in the hope that his Restoration would finally return Catholic rights and land.”

Through his research he discovered and identified the British flag taken by the Irish Brigade at the Battle of Fontenoy.

He says, "There has always been controversy about this flag as it was never correctly identified. But I came across a reference to it in a French newspaper from 1745 and was finally able to track it down in an illustrated hand-painted manuscript in the research library in Paris. If I ever was to have a ‘eureka’ moment – it was now."

McGarry estimates that 50,000 Irishmen joined the brigades in France and Spain and said that during the 18th century many men turned to military service abroad due to the lack of opportunities for Catholics under the Penal Laws.