This has been discussed a few different times before. In one thread on historical films, here are my brief comments. Another thread on it though I disagree slightly with /u/Bernardito (to me, the biggest issue is the Communist-theory bias of the film, but that is more my issue has perceived by knowledge of the director). And, lastly, another thread only involves me and the OP, but I will paste my response here:

Brilliant movie, and it gets most things right. The brutality of not only the Black and Tans but the Irish Nationalists is well documented through newspapers, letters, diaries, court records, and oral accounts. The historical issues with this film are not how such crimes are represented, but in how Ken Loach and his production crew handle the controversy of this time period. While we know these events took place, there is dispute as to the specifics or who the participants often were. The key representation of this in the movie is the Kilmichael Ambush: the film inaccurately shows how the ambush played out, how many casualties there were, and even the time of day (it actually took place in the evening). While one of the key moments in the movie, the Ambush has been a point of contention among historians of the period (details of which can easily be found here).

One of the other issues with the movie is Loach's Communist perspective. Now, a Marxist approach to history is not a problem, and has been debated through various Discussion Thread's in this very sub, but Loach treats all of the the anti-Treaty Nationalists as being Socialist. While Socialism was a practical philosophy in Ireland it was not the end-all-be-all as the movie makes out.

A great essay on the movie can be found in The Dublin Review and it goes into far more detail than I am able to at the moment. I will note that while it is a well-written and argued essay, I strongly disagree with a few of the author's points regarding History on Film. The essay seems to take the argument, if I remember correctly, that there are only a few "facts" that the audience can take from the movie. I am of the strong view that cinema offers much more than just mere "facts" in narrative: I follow the arguments of Robert Rosenstone and Natalie Zemon Davis that film can be truly accurate in all aspects, which includes representing the minutiae of daily lives and the specific & often unexplored details of any period. In this view of film as history I will stand by The Wind That Shakes the Barley as a worthy movie to watch and gain an appreciation of this tumultuous period in Irish history. Like all texts it has a bias that when understand can still allow for value of knowledge.

This movie gets a great deal right, particularly the points you are interested in. Just be aware that Loach is presenting this history in a manner which best represents his understanding of what occurred and why.

Well the anti-treaty side lost the civil war but were in government within a decade. Trying to portray the civil war as a left v right conflict is Loach imposing his own preferred narrative on Irish history. It is also very close to attempts by modern Republicanism to delegitimise the Irish state by suggesting it was a counter revolutionary outcome. Just to be clear I have no idea what Loach thinks of that view. It means that the film is part of modern debate. I still think it is the best film about the Irish revolution by some way.