The Black and Tans started up in Northern Ireland on the 25 March, 1920 and where ex servicemen where brought in to assist the RIC.
"The British advertised for men willing to "face a rough and dangerous task", helping to boost the ranks of the Royal Irish Constabulary (RIC) in policing the increasingly hostile Irish population. Black and Tans was the nickname given to this special force used to fight the Sinn Feiners; the name came from a well-known pack of hounds in Limerick. They wore uniforms of khaki with black hats and belts, and were RIC reservists, recruited in England, mainly from ex-servicemen. There were about 8000 of them."
As for the drink, many consider the classic Black & Tan to be Bass Pale Ale and Guinness. There is also the half and half, which is another name for a Black & Tan. However, in many North American Irish pubs, the Bass/Guinness combination is called a Black & Tan, while the Harp(lager)/Guinness combination is called a Half-and-Half. (Harp's is brewed by Guinness at Dundalk)