Loneliness
The main theme of the novel is explored by the idea of a little girl raised without parents, because the parents had to flee and wrongfully believed the girl was dead, when she was just hiding. The premise is far-fetched, but meaningful. The girl is an archetypal instance of loneliness, because she represents the full weight of that emotional experience, and she also has the constant paranoia that maybe she is lonely because she isn't lovable. That part of loneliness is part of what makes the emotion so bitter, and when she tells Ralph that her traumatic loneliness has left her unable to connect romantically, she also teaches about another sad tragedy in loneliness—the intimacy issues that often come from extended periods of isolation.
Political strife and broken homes
The novel points clearly to the IRA's conflict with Britain as the main cause of this family's trauma. The little girl deciding to hide is hardly an issue of responsibility, and Captain Gault has good reason to suspect that he will literally die if he stays. The only true cause of the tragedy is the political strife that broke the home in the first place by displacing Captain Gault.
Intimacy and love
Without a template for intimacy (because she raised herself without family), Lucy can't connect with Ralph, even though she likes him and finds him attractive. She feels that perhaps by connecting with her father, she can reconnect her sense of sincerity and love, but the father has the same intimacy issues for much the same reasons. Thematically, what this indicates is that loneliness causes intimacy issues that often make loneliness even more difficult to surmount.
