Laura just pointed me towards this Wikipedia entry which names an Arthur Francis Benjamin Guinness (1937 – 1992), known as Benjamin, who was chairman of Guinness from the 60s. As far as I can tell, he was the great great great grandson of the original Arthur Guinness who purchased the first brewery in 1759 and was a member of the House of Lords and the 3rd Earl of Iveagh.
People who know me will know that my particular love/obsession with the black stuff makes this quite a weird coincidence!