Galloglas were mercenary warriors from the Hebrides and West Highlands who settled in Ireland in the later 13th century and achieved an extraordinary prominence on Irish battlefields throughout the three hundred years following. Fighting as heavy infantry - highly-disciplined, mail-armoured and wielding their characteristic weapon of the long-staved war-axe - they were the decisive military component in the Gaelic Irish resurgence of the 14th century and represented the cutting-edge of resistance to Tudor reconquest two hundred years later. Found first in the service of native Irish lords in Ulster and Connacht, they were later brought into Munster and Leinster by the gaelicised Anglo-Irish earls. By the 15th century they were established as Ireland's first professional warrior class and, like other professional classes in the Gaelic world, they were organized on the basis of kin-group.

The names of hereditary commanders of galloglas entered in the Irish annals identify these mercenary warrior kindreds as the MacCabes, MacDonnels, MacDowells, MacRorys, MacSheehys and MacSweeneys, all of them families descended from the Gaelic-Norse aristocracy of Argyll and the Isles - and yet their story has been called "a forgotten chapter of West Highland history". This account of the Galloglas is written from a decidedly Scottish perspective, tracing the origins of six kindreds and investigating the circumstances which brought about their relocation to Ireland. It goes on to examine the galloglas as warriors, pointing to their distinctly Norse character and proposing their battle-fury as "the last unmistakable echo of the Scandinavian impact on the Celtic west".