What were the Black and Tans supposed to do and what did they actually do?

Soon after the end of WW1 Ireland exploded into civil war. The 'Black and Tans' is a nickname for a group of paramilitaries recruited by the British government to fight against the Irish Nationalists (1920-22). They acquired a reputation for appalling brutality and thuggery, including torture, arson and murder. Among other things they often exacted revenge. This sometimes took the form of burning down villages and small towns. It's often said that many of the Black and Tans were men who had been brutalized by WW1 and were unable to return to civilian life.