Are you Dunn?

Old joke has the following lines:

Are you Dunn?
Yes.
Then write your mother in Ireland!

Anyone know the whole joke?

[Isn't that the whole joke? What more is required? - Bac.]

Mrs. Dunn: Oh Seamus, when ye get to America, find me son John and tell 'im to write me!
Seamus: Aye Mrs. Dunn, that I'll do!
...But on his arrival in New York, Seamus realizes he has no idea where to find John Dunn. As he walks down the street, he sees a building with a sign that says "Dunn & Bradstreet". Seamus eagerly enters the building, asks the receptionist if they have a "John". She directs him to a little room around the corner. He knocks, and says...