Black and tan is customarily done by mixing two beers together Harp and Guiness. when poured together properly they separate leaving the dark beer on the bottom and a lighter beer on top and then when the class is tilted and drank both parts flow onto your palate.
So during the ceremony they blend of the two together and drank it.
Then similar to the wine box and love letter ceremony they placed a bottle of Harp and Guiness into a box along with some love letters that they wrote the night before.
everything in the ceremony just turned out so wonderful and special and the couple and all the guests loved it.