Two lifelong friends find themselves at an impasse when one abruptly ends their relationship, with alarming consequences for both of them.
Although the lied is actually by Brahms, having such sad Schubertian music in a film with a donkey laden with symbolism is perhaps meant to evoke Robert Bresson's Au Hasard Balthazar? Difficult to improve on Mark O’Connell's view in Slate, however. Take him simply on the Civil War allusion:
As a metaphor it’s both vague and clumsy; for it to work, you’d have to think of the Irish Civil War as some kind of basically unfathomable squabble between former best friends, as opposed to a conflict over a treaty with the British government that granted only partial independence and divided Ireland into two political entities, to disastrous results. As a political allegory, it seems obviously retrofitted, tacked onto the narrative to add unearned resonance.
As for the twentysomething village-idiot Dominic, played by Barry Keoghan, he’s an example of McDonagh’s most cynical dramatic instincts—a character styled for the majority of the film as a walking punchline, before being transformed into a locus of pathos.
— Adam Nayman (The Ringer)