What is the name of the group performing atrocities in "The Wind That Shakes the Barley"?

The group performing atrocities in "The Wind That Shakes the Barley" was the Black and Tans.

Ken Loach’s 2006 film is set primarily in County Cork during the Irish War of Independence, from 1919 to 1921. The Black and Tans were temporary constables recruited to reinforce the Royal Irish Constabulary, the police force operating under British rule in Ireland. Their nickname came from the dark police-style tunics they wore with khaki military trousers.

In the film’s opening sequence, Black and Tans soldiers execute the young hurler Micheál Ó Súilleabháin after he refuses to give his name in English and strikes a British officer. The incident shocks Damien O’Donovan, a medical student, and helps drive his decision to join the Irish Republican Army.

The Black and Tans are sometimes confused with the Auxiliary Division, another British-recruited force active during the conflict. Both units were associated with reprisals and violence, but the film specifically identifies the soldiers in this scene as Black and Tans. The movie later follows the brothers’ conflict into the Irish Civil War.