Uniform Question

The two what? Black and tans was a nickname for the Royal Irish Constabulary Auxilary force. They were the force.

Good guys may not finish last but they sure as sh*t don't finish first!

British Soldiers played a minor supporting role in the 1919-21 period.
Martial Law was not always used and where it was it was limited. They concept of martial law in the UK is/was perceived as a failure of UK society, which is not the same as the failure in a colonial context. Ireland was NOT a colony.
In fact it was used in the part of Ireland covered by this film. General Pecival in command in Cork, later surrendered to the Japanese in Singapore in 1942.
The British army uniform is well documented and not amended in any way in Ireland.

The Black and Tans wore a combination of Police Navy Blue and Khaki green. Trousers and Tunics were always reversed. Apparently to save cost on uniforms and to use up the existing stocks. They wore a Tam O'Shanter type beret. In fact a type of beret favored by an Irish born officer serving in Ireland at the time called Field Marshal Montgomery.
They were dubbed the "Black & Tans" after a pack of hunting hounds in Tipperary as a derogatory term.

Auxiliary uniforms were usually British Army Officer type as they were a "special force" recruited from the demobbed WW1 officer corps. One of the early "Elites" in fact. The ambush in the film replicates an event in west Cork where 18 Auxies were taken down by Tom Barry. Cork city was burned down by the "Tans" after that one.