Some of these are chapters in larger books:

The World of the galloglass: war and society in the North Irish Sea region, 1150-1600, ed. Duffy, S (Dublin 2007)

MacGregor, M, ‘Warfare in Gaelic Scotland in the Later Middle Ages’, in A Military History of Scotland, eds. Crang, JA, Spiers, EM and Strickland, M (Edinburgh 2010)

Simms, K, From Kings to Warlords: The Changing Political Structure of Gaelic Ireland in the Later Middle Ages (Bury St. Edmonds 1987; paperback edition 2000)