Friendships can be as changeable and temperamental and outright dramatic as grand romances, though they tend to get a bland rap on screen — with friends, for most screenwriters, merely convenient constants, there to support protagonists through matters of supposedly more consequence. If substantial platonic relationship studies are rare, ones about men are rarer still. And if that comes down to a social convention rather than a cinematic one, that’s integral to the power and poignancy of Martin McDonagh’s searing “The Banshees of Inisherin,” a film that traces the tortured breakup between two best pals in remote rural Ireland with all the anguish and gravity of the most charged romantic melodrama — its high, unleashed emotions all the more startling in a world where men don’t speak their feelings.

Set in a conservative, harshly patriarchal island community in 1923 — a forbidding body of water away from the mainland, where the Irish Civil War drags grimly on — “The Banshees of Inisherin” mostly sticks to that world, peopled as it is with surly, taciturn men, women encouraged to melt into the stonework and livestock hardly less expressive than their human minders. When its characters break and vent and hold forth, however, they do so in the ornately verbal, gruffly poetic and violently hilarious vernacular of McDonagh’s best writing — regaining, after two American-set efforts, the Irish brogue of both his heritage and his splendid 2008 debut “In Bruges,” whose stars Colin Farrell and Brendan Gleeson are once more ideally paired here. The result feels closer than any of his previous films to the barbed, intimate lyricism of McDonagh’s work as a playwright, and more deeply, sorrowfully felt to boot.

In “In Bruges,” the tension between Farrell’s laddish puppy-dog restlessness and Gleeson’s more seasoned, world-weary calm fueled a buddy movie in which partnership was hard-won from frosty beginnings; “The Banshees of Inisherin” pulls the reverse move, showing a close friendship disintegrating back to a state of poles-apart disparity between the two leads. We are never given a glimpse of gentle-natured dairy farmer Pádraic (Farrell) and pensive older musician Colm (Gleeson) in cheerier times. McDonagh instead opens on the day Pádraic drops by Colm’s whitewashed clifftop cottage for a shared stroll to the village pub — as they’ve done every afternoon for eons — and finds, to his bemusement, that his friend won’t join him, won’t even come to the door, and won’t explain why.

For Pádraic, a simple but sensitive type, this snub reduces his social circle to a mere dot — perhaps a short line if you include amiable village idiot Dominic (Barry Keoghan), which nobody really does. Orphaned and unmarried, Pádraic shares his parents’ scruffy old home with his beloved donkey Jenny and his older sister Siobhan (a revelatory Kerry Condon), a nurturing, bookish woman who has never really found her people on this desolate, unkind island. It’s a protective Siobhan who manages to tease out of Colm the reason for his abrupt termination of his and Pádraic’s friendship: he finds the younger man dull, has more or less run out of things to say to him, and would prefer the company of his fiddle and his devoted border collie.

Unsurprisingly reluctant to take such an explanation lying down, Pádraic decides he’s been a casualty of Colm’s escalating depression, and brightly resolves to claw his way back into his ex-friend’s affections. His charm offensive is halted, however, when Colm issues a macabre ultimatum that vaults a simple estrangement to the level of an eccentric two-man blood feud. What begins as a doleful, anecdotal narrative becomes something closer to mythic in its rage and resonance: McDonagh has long fixated on the most visceral, vengeful extremes of human behavior, but never has he formed something this sorly heartbroken from that fascination. 

There’s much talk here of “niceness,” which has never been this filmmaker’s default setting: Pádraic prides himself on it, while Colm, whose had a decade or so longer on the planet to tire of social niceties, has come to see it as an overrated virtue. McDonagh’s script has sympathy for both, while audiences may find themselves intriguingly split. There’s a kind of admirable, self-knowing integrity in Colm’s simple, increasingly obsessive desire to be alone; Pádraic’s terror of being left alone himself, especially as Siobhan wistfully eyes a life beyond the island, is just as understandable. Condon, wry and warm but no twinkly, benevolent cypher, makes Siobhan the one character who can credibly empathize with both men.

Farrell and Gleeson’s lovely, perfectly mismatched performances, meanwhile, both betray their own manner of gaping, aching vulnerability. The former’s caterpillar eyebrows have never seemed more boyishly worried, his open smile never more hopeful or eager to please: He walks the grassy paths he’s covered every day of his life with a purposeful stride that suggests he might yet find something new in them. Gleeson’s burdened, exhausted posture and perennially fallen expression, on the other hand, are the tells of a man who long ago stopped looking. “How’s the despair?” the local priest asks him as he slumps into the confession booth. “It’s back a bit,” Colm shrugs. “I’m not going to do anything about it.”

After a teasingly postcard-bright intro — which sets up an Emerald Isle ideal of verdant fields, rainbows and sunlight skittering across the ocean, soon to be bluntly shattered — McDonagh crafts an Ireland where despair, for everyone, is something to be managed rather than beaten. Ben Davis’s lensing washes even the characters’ best days in raincloud grays; Mark Tildesley’s production design trades in cramped, muddy spaces shorn of ornamental detail. 

It makes for a story world seemingly drained of tenderness, in which every character is either single, widowed or otherwise alone; Pádraic and Colm’s now-bloodied friendship was perhaps the purest thing in it. As Colm insists to the priest that he’s never had “impure thoughts about men,” its tempting to consider a queer undertow to the bond that was, though the truth is that the two warring men never seem much like soulmates — simply the next best thing on a isle short of souls. It’s the loss even of such modest mercies that makes McDonagh’s quietly magnificent film so caustically, hauntingly and sometimes raucously sad.