The name itself is derived from a political reference to the black and khaki military uniforms worn by the special auxiliary force - 'The Black and Tans' - who were brought in to Ireland fight the Irish nationalists in 1920.
Normal practice for making a Black and Tan is to first pull half a pint of ale into a pint glass, and then, using a spoon placed under the tap, to roll the stout oh-so-gently on to the top of the ale layer.
Done properly, the procedure produces a Black and Tan; black stout on the top and tan ale, or lager, underneath. Brownian Motion aside, the two beers won't mix in the glass until they are disturbed, for example, by drinking.
A classic Black and Tan consists of a foundation of Bass Pale Ale topped by thick black Guinness, the combination of English and Irish beers somehow cementing the origin of the name.