Guinness Draught Bottles and Harp Lager are widely available. You may also combine the beers without "floating" the Guinness.
1 (11.2-ounce) bottle cold Guinness Draught
1 (11.2-ounce) bottle cold Harp Lager
Vigorously (so it develops a good head) pour half a bottle of Harp into each of the two 12-ounce or larger glasses. Slowly add the Guinness, pouring it over the back of a spoon held at an angle into the glass. The light and dark beers will stay separated for some time.