This film brilliantly portrays a little-known historical subject. Many know of the brutality of British colonialists in India (thanks to Gandhi) or Africa, but they were just as harsh towards the neighboring Irish as well. Thanks to the film, more viewers are able to see that the British Empire's brutality was experienced by their closest neighbors.
Ultimately, what the movie shows is that, in many ways, the British come out on top in the end following the signing of the treaty. What the English couldn't successfully do before the treaty (destroy the pride and independent spirit/desire of the Irish people) was what they were largely able to achieve following the document's ratification. The Irish were now fighting their fellow Irish, and the British no longer had to waste any of their soldiers to do so.
Plot-wise, what especially struck me was the scene where Damien checks on the sick little boy in bed. Despite abandoning his medical career to join the IRA and a life of violence, Damien demonstrates here that he still has love and compassion deep down, and that the doctor within him, while suppressed, is not extinguished.