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As with his 1995 “Land and Freedom,” set during the Spanish Civil War, Ken Loach melds historical and personal drama in this film, situated in Ireland during the troubled days of 1920-21, when the IRA was formed and the British government was forced to agree to the division of the island—a situation that’s prevailed up to the present and prolonged the country’s suffering. Against this backdrop Paul Laverty’s script tells the story of two brothers, Damien and Teddy O’Donovan (Cillian Murphy and Padraic Delaney) who join the IRA together but split company after the treaty of 1921 established the so-called Irish Free State, which remained a dominion of England while allowing the British to continue the direct rule of what became known as Northern Ireland.

Damien is initially the less bellicose of the two, planning to go off for a medical internship in London while Teddy and his pals plan on resisting English occupation and the mindless brutality of the so-called Black and Tans. He decides to remain, however, after he witnesses the killing of a young man who can’t give soldiers his name in English, and the beating of Dan (Liam Cunningham), a trainman who refuses to transport occupying troops on his route. Soon the brothers and their colleagues are stealing weapons from the local constabulary and shooting soldiers; in one especially powerful episode, they’re also captured and tortured by the British but escape, only to take vengeance on the neighboring lord and his young worker who turned them in. Damien also becomes romantically involved with Sinead (Orla Fitzgerald),

The peace agreement of 1921, however, divides the siblings, with Teddy believing it a step in the right direction and joining the Free State police force, while Damien rejects it as insufficient and resumes guerrilla tactics against the new regime. The outcome is, in line with the bloody nature of Irish history in the last century, predictably tragic.

There’s much that’s excellent in “The Wind That Shakes the Barley.” Loach and cinematographer Barry Ackroyd, along with production designer Fergus Clegg, art directors Mark Lowry and Michael Higgins and costume designer Eimer Ni Mhaoldomhnaigh, have achieved an exquisite period look, making fine use of outdoor locations and existing structures to do so. Murphy is extraordinarily fine as Damien, especially in the harrowing scene when he must overcome his natural inclinations to execute two prisoners. Though they don’t match him, Delaney and Cunningham are also fine, and the supporting cast is filled with memorable faces who may have limited screen time but make an considerable impression.

What weakens the film is Loach’s customary penchant for political didacticism, which becomes increasingly pronounced in the second half. A scene focusing on an argument between the military leadership and a court the IRA has set up in a liberated village is a springboard for opening the issue of the rule of law—an interesting topic, but one that’s very schematically outlined. And a heated discussion among the local IRA leadership about the pros and cons of the 1921 treaty has a debate society quality about it, although admittedly the points are made with dramatic impact. One senses Loach’s sensibility, moreover, in the intransigent position taken in the end by Damien, who stands up for the ideal of an independent and unified socialist Ireland and accuses the church, which supports the Free State, of once again “siding with the rich.” All the political talk means that short shrift is given to the Damien-Sinead subplot, which means that Fitzgerald’s character isn’t fleshed out as fully as it might have been. And the brother-versus-brother plot does, of course, have a ring of familiarity from similar tales not only of civil war but of gangster life.

Still, though Loach’s film loses some of its dramatic impetus in a talky second half, it illuminates the events that led to eight decades of tortured Irish history with sufficient skill to be worth seeing.