What is ironic about the scene with the Sergeant and the Ragged Man?

Isabella Augusta Persse introduces irony in two main ways during this scene. In different ways, both the Sergeant and the Ragged Man are disguised and then reveal their true selves. In addition, the two characters reverse their functions: the Sergeant begins as the pursuer and investigator but ends being investigated and found out by the Ragged Man.
The Sergeant is metaphorically disguised because he is hiding his true feelings about the Irish nationalist cause. As the two men converse, these feelings emerge at first when he admits knowledge of the song lyrics. The Ragged Man is literally disguised as the singer and ballad seller and finally reveals his identity as the wanted man. The Sergeant’s sympathies get the better of him when he lets the man escape.
The main reason the Sergeant was on the wharf was to keep watch for the wanted man. He allows the man first to distract him and then to pry out his true sentiments. The Ragged Man was on the run, trying to escape from the policemen. He patiently waits in a dangerous situation while ferreting out information that could be—and indeed proves to be—useful to him.

One realizes the irony of the situation of the Sergeant and the Ragged Man in The Rising of the Moon as they sit talking on the barrel overlooking the river. The Sergeant is a poor family man who ekes out a living, the Ragged Man is an Irish diplomatic patriot who disguises himself as a poor man. The Ragged Man begins to sing an Irish ballad which brings back memories to the policeman. In their discussion, the Sergeant reveals that as a youth he had patriotic feelings and divulges this information to the patriot when he misses a line in the ballad he is singing. The sergeant acknowledges that if his life circumstances were different that he might have become a patriot for the Irish cause himself.
It is ironic that the Ragged Man has actually disguised himself to be a poor man eking out a living singing his patriotic ballads for the sailors, while the sergeant who had been a patriotic at heart as a youth, tries to capture the escapee.