Depends on how you look at it, Simon. Found this online...
What is a Black and Tan?
The answer depends on who you ask. The all-Guinness Black & Tan is Harp's Lager and Guinness. (Harp's is brewed by Guinness at Dundalk). Many consider the classic Black & Tan to be Bass Pale Ale and Guinness. (To some Irish, they appreciate the fact that the Irish comes out on top!) However, Bass Ale is not as available in Ireland as it used to be, and either Harp or Smithwicks is generally used. However, there are many variations involving a stout or porter and another lighter coloured beer, either lager or ale.
A half-and-half is often just 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/Guinness combination is called a Half-and-Half.
I use Harp because of how light it is. Haven't tried Bass Pale, though. Didn't know it existed. Plus, I have a glass I stole from a pub that says Black and Tan on it with the logos for Guinness and Harp. Now that I know that Harp is brewed by Guinness, the conflicting reports make more sense; Guinness wants to sell all their own brands so they'll shill Harp as the tan on their own glasses.