The best and most readable account of the Black & Tans that I have yet encountered is "The Black & Tans", by Richard Bennett, formerly a Lt.Col in the British War Office. ISBN 1-566190-820-8. It's available from Barnes & Noble for about US$10. Conditions in Ireland at the time were tumultous, to say the least, and Britain was experiencing a post-war economic depression and severe unemployment. Lloyd George's way of addressing both problems was to recruit unemployed british veterans for police service in Ireland. Those later known as "Black & Tans" were former enlisted men, and those known as "Auxies" were former officers. These men were to bolster the RIC, and together they were backed by a force of about 43,000 British troops in Ireland. In addition to a payroll, the B&Ts and Auxies also were granted a modernized version of the ancient right of plunder.

I would be very cautious about anything written by Conor Cruise O'Brien. Aside from being the darling of Ian Paisley, he is generally recognized as a revisionist and apologist, or worse, depending on the company. Make your own judgments, but be aware that his academic objectivity is suspect.