As it turns out, the harp on the glass has more than ornamental purposes. When Guinness is poured properly into a branded Guinness pint glass, the harp icon serves as a measuring aid.
Back at the Guinness Storehouse, the fourth floor is home to the Guinness Academy, where you can learn the art of the perfect pour. Start the pour by holding the glass at a 45-degree angle under the spout and slowly fill the glass to the top of the harp. Allow the beer to settle before topping it off.
The harp has been a widely recognized beacon of the Guinness brand since 1876. It is based on the harp of Brian Boru (housed at Trinity Library), the high King of Ireland from 1002 to 1014.
Coincidentally, a harp is also the official national emblem of the Republic of Ireland, appearing on passports and state documents. The Guinness harp always has a Left straight edge and the Republic of Ireland harp always has a Right straight edge in order to distinguish the two trademarks.
Awfully nice of the Country to adjust their national symbol to accommodate the trademark of a (albeit beloved and iconic) beer brand!