Michael tells Cathleen that he wants to help in whatever way he can, but Peter and Bridget try and dissuade him from getting too invested in her message. Cathleen warns Michael that those who help her have a hard time of it, and many have lost their lives. As she leaves, Cathleen sings a song about the rewards of fighting for Ireland, the immortality and honor that it brings to a person.
After she leaves, Bridget notes to Peter that Michael looks like he's lost his mind. She then tells him to go try on his wedding clothes, to make sure they fit for the next day. "What wedding are you talking of?" Michael asks, confused, forgetting that he is getting married to Delia Cahel.
Suddenly, there is the sound of cheers and shouting outside, as Patrick and Delia come in. Patrick tells them that "There are ships in the Bay; the French are landing at Killala!" Delia calls to Michael, but he barely recognizes her. Patrick tells them that the boys are going down the hill to join the French.
Delia suggests that Michael will not be joining the French and Bridget tries to get Peter to reason with him, but Michael cannot even hear them. As Delia embraces Michael, begging him to marry her, we hear the Old Woman's voice outside, as she repeats the lines from before: "They shall be speaking for ever,/The people shall hear them for ever."
Michael leaves, and Peter asks Patrick if he saw an Old Woman on the path. Patrick replies, "I did not, but I saw a young girl, and she had the walk of a queen."
The price of joining Cathleen's cause and fighting on behalf of Irish nationalism is a steep one. Cathleen warns Michael that, "Many that are red-cheeked now will be pale-cheeked; many that have been free to walk the hills and the bogs and the rushes will be sent to walk hard streets in far countries...many a child will be born, and there will be no father at its christening to give it a name." Here, we see that the revolution being staged by the Irish nationalists is not a simple solution and will involve great sacrifice and personal tragedies.
If the price is high, then the rewards are also high, as Cathleen sings on her exit. She tells Michael, "They [the people who fight on her behalf] shall be remembered for ever,/They shall be alive for ever,/They shall be speaking for ever,/The people shall hear them for ever." Here, she outlines the fact that to fight for the cause of Irish nationalism is to ensure a kind of immortality, an ennobled belonging in a movement that has historical and ethical significance beyond the earthly realm. She promises a heroic elevation to all who fight on behalf of Ireland.
Cathleen casts a kind of magic spell over Michael upon leaving. After she leaves, he barely remembers the fact that he is getting married the next day and can think only of the patriotic spirit that Cathleen invoked in her visit. He needs only a visit from a charismatic old beggar woman to convince him that the Irish cause is worth fighting for. All of the preparations that have been done for the wedding the next day dissipate from his mind as he realizes that he has no choice but to join the French in fighting for an end to British colonial rule.
Once Michael is under the spell, his thoughts of the wedding to Delia fly out the window. She begs him not to go and fight, but he does so anyway, choosing Ireland over his wife. In this sense, the play shows the ways that men fighting for the Irish nationalist cause had to let go of their lives and leave behind all of the structures and relationships they had made. In this play, Michael barely even recognizes the woman he was preparing to marry, as he so enchanted by Cathleen ni Houlihan and her message.
After having delivered her message, Cathleen transforms into a young girl, as Patrick reports. The image of an Irish nation that is unsupported by the young soldiers of Ireland is thus presented as an old and desperate woman. When a man has pledged his allegiance to Ireland and the cause, however, the nation is blessed with youth and beauty. Patrick says that the young woman he saw on the road "had the walk of a queen," which suggests that Michael's giving himself over to Irish nationalism grants the figure of Cathleen ni Houlihan a sovereign poise and power.