Known perhaps best to the average modern anglophone through their mention in Macbeth - though I personally first heard of them in a wargaming context - the galloglas were a warrior caste of Scottish origin but soon enough nativized in 13th to 16th century Ireland. As professional heavy infantry, at least in their later years characteristically armed with poleaxes, they become indispensable to Irish armies, and eventually also to English ones on the island.

The operative word of the subtitle of this book however isn't so much "warrior" as "kindreds" - the focus is on tracing the half-dozen originally Scottish clans that provided their leaders. Not uninteresting, but not quite what I was expecting either.