Q: What do you call a fake Irish stone?
A: A shamrock!

Explanation: March 17th is St. Patrick’s Day, in honor of the patron saint of Ireland.

A shamrock is a bit of clover, typically with three leaves as you see in the picture.  It is sometimes used as symbol of Ireland.  People say that St. Patrick, a Catholic missionary priest, used it to teach about the Christian Holy Trinity.

If something is a sham, it is fake, not what it is supposed to be.  In the case of this joke, a sham-rock would be a rock (rock and stone are synonyms) that is not really a rock.

This joke is funny because it plays with the word sham and the word shamrock.

The music and dancing below is not a sham, it is Riverdance: