In the play The Rising of the Moon, a Sergeant sits by an Irish wharf keeping a sharp eye out for an escaped prisoner. A man comes up to the Sergeant and identifies himself as a ballad singer named Jimmy Walsh. The singer tells the Sergeant that he knows the prisoner well and that he will help watch for him. The two sit down together and chat. Then Walsh begins to sing a ballad of the Irish rebels (the group to which the prisoner belongs).
The Sergeant tells him to stop. This is not the song to sing in these troubled times. Walsh replies that he is only trying to keep his “heart up.” Then he assures the rather nervous Sergeant that he is keeping “a good look-out” and not even for any reward. Here is where the Sergeant says, “You will get your reward in heaven.”
The Sergeant means that all good deeds, even if they are unrewarded in this life, will eventually be recognized and honored by God in the next life. He is merely trying to assure Walsh that people who do good things will receive good from them in the end.
The line, however, ends up looking extremely ironic by the end of the play. The man who calls himself Jimmy Walsh is actually the escaped prisoner, one of the Irish rebels about whom he sings. Walsh, of course, believes that he is in the right with his activities against the oppressive government, and deep down, the Sergeant does, too, for when he finds out who his companion really is, he decides to let him go. Walsh seems to think that he will indeed get his reward in heaven, even though he is on the run now, and perhaps he believes that the sympathetic Sergeant will, too.