The Irish conflict with Britain
Britain is a historically dominant nation, in that its armies conquered much of the world at the height of the British empire, and Britain's neighbors to the north in Ireland were not immune to that. But, for other places, independence was being achieved, because the British knew that to fight to keep those nations (like America, for instance) was a lost cause. This book shows that, for the Irish, independence was more complicated. The conflict is shown for its political facet and its military facet, but ultimately, although they technically never got quite what they wanted from Britain, the Irish defiantly resisted them.
The problem of violence
For the Irish who fought for independence, there is another looming question (at least in this novel): Can they all defend their violence? Henry notices toward the closing of this novel that, actually, it seems some people have become full-blown terrorists, simply fighting for their point of view—instead of remembering the objective. This is a thematic portrait of the inherent danger of violence. Ultimately, nonviolence is more effective whenever it's an option, but for the Irish, violence made them extreme and volatile, at the expense of their strategy, and perhaps at the expense of their movement.
The problem of imperialism
Ultimately, the major problem in the novel is not the Irish at all—it isn't even the British, technically. Technically speaking, the problem is that, at the geo-political level, imperialism was still tolerated by the global community. By acknowledging the divine sovereignty of all human beings, we can free others to rule themselves according to their own customs. The Irish say they are being afflicted and oppressed by British reign, and when the British came to reinforce their totalitarian imperialistic ways, they fought and died to make the point that British imperialism was morally wrong.
