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: