Katie's War — Aubrey M. Flegg

When Katie's father returns from the Great War, he is shell-shocked, his personality destroyed. Now, four years later he has more or less recovered, but another war is breaking out, this time at home in Ireland. The Treaty with the British has been signed by Michael Collins, but many disagree with it and want to continue the war for full independence. The country is on the brink of civil war. There are divided loyalties in Kate's family, and she has to choose whom she will support. Finally, she and the Welsh boy Dafydd make a bold plan to destroy an arms cache…