Mr. Muldoon has entered the house and has proposed that Caitlin can move into a new house under his direct supervision. It is very tense, and Quinn is insisting she should not agree. But she does. Then, Tom Kettle screams/heaves a bit and storms in, holding Oisin in his arms. Oisin has clear red marks all around his neck.
Tom lays him down center stage, and we see that Oisin is unconscious. Caitlin freaks out and tends to him, while Tom explains that Oisin had snuck into his house, stood over his bed with a gun, and exclaimed that he intended to kill Tom. Tom then admits to stop him, he rung Oisin's neck. Caitlin screams.
The family starts pouring down the stairs, and Caitlin, sobbing, pick herself up and starts rambling/crying "It's ok it's ok it's ok..." while she makes her way to the stage right shelf, where the shaving razor is stored. She grabs it, and makes a run for Mr. Muldoon on the opposite side of the stage. Quinn stops her right before she gets to him, and holds her.
Quinn then takes the razor from her, and in a split second, turns around and slashes Muldoon's throat. Blood spurts out of his neck. He then takes the gun that Tom had confiscated from Oisin and shoots one of Muldoon's henchmen, with a big splat of blood going against the wall. He points the gun to the second henchman, and tells him to run to the city and tell everyone that Quinn Carney has exacted revenge on Seamus Carney, ending the Muldoon run.
Aunt Pat starts freaking out (the whole family is, but she especially) screaming, "What have you done to this family?!" The eldest son asks what they do next.
Maggie Far Away walks in and starts repeating the story she began earlier in the third act, walking toward the window murmuring, "They're coming, they're coming, they're coming" until finally it builds to "they're here, they're here," and then she turns to Quinn and more or less screeches, "THEY'RE HERE."
Blackout.
A lot of tense underscoring is used and the tension is thick. It all happens so fast, but is just so satisfying.