Final answer:
The musical instrument on the Guinness beer label is a wire-strung Irish harp, which is synonymous with the brand and a symbol of Ireland.
Step-by-step explanation:
The musical instrument that appears on the Guinness beer label is the harp. Specifically, it is a wire-strung Irish harp, also known as a cláirseach, which is a traditional symbol of Ireland. This emblem has been associated with the Guinness brand since 1862 and is based on the 'Brian Boru harp' which is on display in the Long Room of the Trinity College Library in Dublin, Ireland.