The dramatic ending of this play is unconventional as well as clever.
To start, your essay would more than likely include a thesis statement. Your question might well serve as a starting point. You hypothesize that the ending is dramatic, therefore, your essay will delineate how Priestley makes his ending dramatic.
Your first paragraph might also include a brief synopsis of the play. The main characters are Arthur Birling (a wealthy industrialist), Sybil Birling (his cold, calculating wife), Sheila Birling (the Birling daughter), Eric Birling (the Birling heir), Gerald Croft (Sheila's fiance) and Inspector Goole (the enigmatic police detective).
The main body of your essay might describe how the Birling family and Gerald Croft have all had an acquaintance with Ava Smith at some point before her death. How does Priestley make the ending dramatic? His Inspector Goole is a surrealistic, avenging-angel type, with one caveat: he does not avenge Ava's death. However, he performs part of the ministrations of an avenging angel. Inspector Goole can be thought of as Ava's lost voice, seeking justice for her mistreatment and marginalization at the hands of the people Goole interrogates. You will notice that Goole seems to know details about the Birling family and Gerald Croft that he couldn't possibly have known unless he was intimately cognizant of the exchanges between them and Ava.
The dramatic ending positions the interrogation as having never happened, just as the Birlings and Croft assert. They are triumphant when phone calls to confirm the identity of Inspector Goole ascertain that there is no such detective and that a suicide has not been reported for months. However, to their horror (and to ours), Ava's death is still to happen: this is what causes the ending to be dramatic. When Birling gets word by phone that Ava has just died precisely the way Inspector Goole previously described, the terror of the characters is palpable (obvious, evident). What recourse might individuals have when they have already incriminated themselves by their previous confessions?
Inspector Goole is the quintessential vigilante-avenging-angel type. He is an outsider, mysterious, grim, and uncompromising. He is there to speak up for the victim and he will tolerate no excuses for guilty actions. The ending would not have been quite so dramatic without Inspector Goole's uncanny questioning and seemingly encyclopedic knowledge of the exchanges between the victim and the guilty parties previous to the terrifying phone call. Like an avenging angel, Inspector Goole pronounces judgment before he leaves.
One Eva Smith is gone- but there are millions and millions of Eva Smiths and John Smiths still left with us, with their lives, their hopes and fears, their suffering and chance of happiness, all intertwined with our lives...We are responsible for each other and I tell you that the time will soon come when if men will not learn that lesson, then they will be taught it in fire, and blood, and anguish.
Unspoken is the final question: will the guilty parties now have to account for their deplorable treatment of Ava through a baptism of fire, blood, and anguish? Dramatic ending, wouldn't you agree?