Insta-Jig

You find a box in the attic.

This box contains a violin in good condition, though clearly used.  Playing any Irish drinking song on this violin will cause everyone in the vicinity to start drinking (beer will materialize as necessary — Guinness, of course) and sing along.

Nota Bene:  Singing and drinking of alcoholic beverages are frowned upon in some cultures.  Also note that everyone will know that the violin and the person playing it are the cause of the activity.