I know the civil war and the occupation of the Black and Tans are considered to be a dark time in Irish history, but as far as crimes committed (by either side) do we have any written evidence or it is all oral accounts?
Sorry if this is a stupid question, I am Irish, but only remember learning that it was a terrible time in school, but as far as I remember there was never any explanation and to be honest I haven't read up on the subject.
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.