Jeremiah Donovan is the officer in charge of the group of Irish rebels. Unlike Bonaparte and Noble, he does not regard the English prisoners as friendly acquaintances.
In this way, you could say that the theme is of duty, Donovan understands his duty and all parts of it, while the others want to do part of their duty but don't want to think about the hard parts.
Donavan is more aware of the reality of the situation thus he doesn't make friendships with the hostages because he knows they might execute them thus he had his duty first. and one of the main themes in the story is "Duty comes first"